diff --git a/docs/release-notes/.FSharp.Compiler.Service/9.0.300.md b/docs/release-notes/.FSharp.Compiler.Service/9.0.300.md
index c352b51148..178d1cf843 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/9.0.300.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/9.0.300.md
@@ -26,6 +26,7 @@
 * Update `Obsolete` attribute checking to account for `DiagnosticId` and `UrlFormat` properties. ([PR #18224](https://github.com/dotnet/fsharp/pull/18224))
 * Remove `Cancellable.UsingToken` from tests ([PR #18276](https://github.com/dotnet/fsharp/pull/18276))
 * Added nullability annotations to `.Using` builder method for `async`, `task` and compiler-internal builders ([PR #18292](https://github.com/dotnet/fsharp/pull/18292))
+* Warn when `unit` is passed to an `obj`-typed argument  ([PR #18330](https://github.com/dotnet/fsharp/pull/18330))
 * Warning for "useless null handling" works with piped syntax constructs now ([PR #18331](https://github.com/dotnet/fsharp/pull/18331))
 
 ### Breaking Changes
diff --git a/docs/release-notes/.Language/preview.md b/docs/release-notes/.Language/preview.md
index d6ef41c3d5..905e086a16 100644
--- a/docs/release-notes/.Language/preview.md
+++ b/docs/release-notes/.Language/preview.md
@@ -4,6 +4,7 @@
 * Deprecate places where `seq` can be omitted. ([Language suggestion #1033](https://github.com/fsharp/fslang-suggestions/issues/1033), [PR #17772](https://github.com/dotnet/fsharp/pull/17772))
 * Added type conversions cache, only enabled for compiler runs ([PR#17668](https://github.com/dotnet/fsharp/pull/17668))
 * Support ValueOption + Struct attribute as optional parameter for methods ([Language suggestion #1136](https://github.com/fsharp/fslang-suggestions/issues/1136), [PR #18098](https://github.com/dotnet/fsharp/pull/18098))
+* Warn when `unit` is passed to an `obj`-typed argument  ([PR #18330](https://github.com/dotnet/fsharp/pull/18330))
 
 ### Fixed
 
diff --git a/src/Compiler/Checking/ConstraintSolver.fs b/src/Compiler/Checking/ConstraintSolver.fs
index 2578107176..a45bb37234 100644
--- a/src/Compiler/Checking/ConstraintSolver.fs
+++ b/src/Compiler/Checking/ConstraintSolver.fs
@@ -3177,6 +3177,7 @@ and ArgsMustSubsumeOrConvert
     trackErrors {
         let g = csenv.g
         let m = callerArg.Range
+        let callerTy = callerArg.CallerArgumentType
         let calledArgTy, usesTDC, eqn = AdjustCalledArgType csenv.InfoReader ad isConstraint enforceNullableOptionalsKnownTypes calledArg callerArg
 
         match eqn with 
@@ -3188,8 +3189,10 @@ and ArgsMustSubsumeOrConvert
         match usesTDC with 
         | TypeDirectedConversionUsed.Yes(warn, _, _) -> do! WarnD(warn csenv.DisplayEnv)
         | TypeDirectedConversionUsed.No -> ()
-        do! SolveTypeSubsumesTypeWithReport csenv ndeep m trace cxsln (Some calledArg.CalledArgumentType) calledArgTy callerArg.CallerArgumentType
-        if calledArg.IsParamArray && isArray1DTy g calledArgTy && not (isArray1DTy g callerArg.CallerArgumentType) then 
+        do! SolveTypeSubsumesTypeWithReport csenv ndeep m trace cxsln (Some calledArg.CalledArgumentType) calledArgTy callerTy
+        if g.langVersion.SupportsFeature(LanguageFeature.WarnWhenUnitPassedToObjArg) && isUnitTy g callerTy && isObjTyAnyNullness g calledArgTy then
+            do! WarnD(Error(FSComp.SR.tcUnitToObjSubsumption(), m))
+        if calledArg.IsParamArray && isArray1DTy g calledArgTy && not (isArray1DTy g callerTy) then 
             return! ErrorD(Error(FSComp.SR.csMethodExpectsParams(), m))
         else 
             return usesTDC
diff --git a/src/Compiler/FSComp.txt b/src/Compiler/FSComp.txt
index ad95dc6556..eedaecbb9b 100644
--- a/src/Compiler/FSComp.txt
+++ b/src/Compiler/FSComp.txt
@@ -1683,6 +1683,7 @@ forFormatInvalidForInterpolated4,"Interpolated strings used as type IFormattable
 3394,parsNewExprMemberAccess,"This member access is ambiguous. Please use parentheses around the object creation, e.g. '(new SomeType(args)).MemberName'"
 3395,tcImplicitConversionUsedForMethodArg,"This expression uses the implicit conversion '%s' to convert type '%s' to type '%s'."
 3396,tcLiteralAttributeCannotUseActivePattern,"A [<Literal>] declaration cannot use an active pattern for its identifier"
+3397,tcUnitToObjSubsumption,"This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\"."
 3401,ilxgenInvalidConstructInStateMachineDuringCodegen,"The resumable code construct '%s' may only be used in inlined code protected by 'if __useResumableCode then ...' and the overall composition must form valid resumable code."
 3402,tcInvalidResumableConstruct,"The construct '%s' may only be used in valid resumable code."
 3501,tcResumableCodeFunctionMustBeInline,"Invalid resumable code. Any method of function accepting or returning resumable code must be marked 'inline'"
@@ -1794,3 +1795,4 @@ featureDontWarnOnUppercaseIdentifiersInBindingPatterns,"Don't warn on uppercase
 3874,tcExpectedTypeParamMarkedWithUnitOfMeasureAttribute,"Expected unit-of-measure type parameter must be marked with the [<Measure>] attribute."
 featureDeprecatePlacesWhereSeqCanBeOmitted,"Deprecate places where 'seq' can be omitted"
 featureSupportValueOptionsAsOptionalParameters,"Support ValueOption as valid type for optional member parameters"
+featureSupportWarnWhenUnitPassedToObjArg,"Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`."
diff --git a/src/Compiler/Facilities/LanguageFeatures.fs b/src/Compiler/Facilities/LanguageFeatures.fs
index 5401ea26a1..9ee90f98bb 100644
--- a/src/Compiler/Facilities/LanguageFeatures.fs
+++ b/src/Compiler/Facilities/LanguageFeatures.fs
@@ -98,6 +98,7 @@ type LanguageFeature =
     | UseTypeSubsumptionCache
     | DeprecatePlacesWhereSeqCanBeOmitted
     | SupportValueOptionsAsOptionalParameters
+    | WarnWhenUnitPassedToObjArg
 
 /// LanguageVersion management
 type LanguageVersion(versionText) =
@@ -227,6 +228,7 @@ type LanguageVersion(versionText) =
                 LanguageFeature.DontWarnOnUppercaseIdentifiersInBindingPatterns, previewVersion
                 LanguageFeature.DeprecatePlacesWhereSeqCanBeOmitted, previewVersion
                 LanguageFeature.SupportValueOptionsAsOptionalParameters, previewVersion
+                LanguageFeature.WarnWhenUnitPassedToObjArg, previewVersion
             ]
 
     static let defaultLanguageVersion = LanguageVersion("default")
@@ -388,6 +390,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.UseTypeSubsumptionCache -> FSComp.SR.featureUseTypeSubsumptionCache ()
         | LanguageFeature.DeprecatePlacesWhereSeqCanBeOmitted -> FSComp.SR.featureDeprecatePlacesWhereSeqCanBeOmitted ()
         | LanguageFeature.SupportValueOptionsAsOptionalParameters -> FSComp.SR.featureSupportValueOptionsAsOptionalParameters ()
+        | LanguageFeature.WarnWhenUnitPassedToObjArg -> FSComp.SR.featureSupportWarnWhenUnitPassedToObjArg ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =
diff --git a/src/Compiler/Facilities/LanguageFeatures.fsi b/src/Compiler/Facilities/LanguageFeatures.fsi
index 9057288800..410a8b193c 100644
--- a/src/Compiler/Facilities/LanguageFeatures.fsi
+++ b/src/Compiler/Facilities/LanguageFeatures.fsi
@@ -89,6 +89,7 @@ type LanguageFeature =
     | UseTypeSubsumptionCache
     | DeprecatePlacesWhereSeqCanBeOmitted
     | SupportValueOptionsAsOptionalParameters
+    | WarnWhenUnitPassedToObjArg
 
 /// LanguageVersion management
 type LanguageVersion =
diff --git a/src/Compiler/xlf/FSComp.txt.cs.xlf b/src/Compiler/xlf/FSComp.txt.cs.xlf
index 30a20d94b2..74894da5d0 100644
--- a/src/Compiler/xlf/FSComp.txt.cs.xlf
+++ b/src/Compiler/xlf/FSComp.txt.cs.xlf
@@ -627,6 +627,11 @@
         <target state="new">Support ValueOption as valid type for optional member parameters</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureSupportWarnWhenUnitPassedToObjArg">
+        <source>Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</source>
+        <target state="new">Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureUnionIsPropertiesVisible">
         <source>Union case test properties</source>
         <target state="translated">Vlastnosti testu případu sjednocení</target>
@@ -1712,6 +1717,11 @@
         <target state="translated">Neočekávaný typ funkce v definici pole případu sjednocení. Pokud chcete, aby pole bylo funkcí, zvažte zabalení signatury funkce závorkami, např. | Případ a -&gt; b na | případ (a -&gt; b).</target>
         <note />
       </trans-unit>
+      <trans-unit id="tcUnitToObjSubsumption">
+        <source>This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</source>
+        <target state="new">This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</target>
+        <note />
+      </trans-unit>
       <trans-unit id="tcUsingInterfaceWithStaticAbstractMethodAsType">
         <source>'{0}' is normally used as a type constraint in generic code, e.g. \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\". See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3536\"' or '--nowarn:3536'.</source>
         <target state="translated">'{0}' se obvykle používá jako omezení typu v obecném kódu, například \"T when ISomeInterface&lt;'T&gt;\" nebo \"let f (x: #ISomeInterface&lt;_&gt;)\". Pokyny najdete v https://aka.ms/fsharp-iwsams. Toto upozornění můžete zakázat pomocí #nowarn \"3536\" nebo '--nowarn:3536'.</target>
diff --git a/src/Compiler/xlf/FSComp.txt.de.xlf b/src/Compiler/xlf/FSComp.txt.de.xlf
index 99eb6f814b..7740961586 100644
--- a/src/Compiler/xlf/FSComp.txt.de.xlf
+++ b/src/Compiler/xlf/FSComp.txt.de.xlf
@@ -627,6 +627,11 @@
         <target state="new">Support ValueOption as valid type for optional member parameters</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureSupportWarnWhenUnitPassedToObjArg">
+        <source>Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</source>
+        <target state="new">Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureUnionIsPropertiesVisible">
         <source>Union case test properties</source>
         <target state="translated">Eigenschaften von Union-Falltests</target>
@@ -1712,6 +1717,11 @@
         <target state="translated">Unerwarteter Funktionstyp in Felddefinition von Union-Fall (case). Wenn das Feld eine Funktion sein soll, sollten Sie die Funktionssignatur in Klammern einschließen, z. B. | Case of a -&gt; b als | Case of (a -&gt; b).</target>
         <note />
       </trans-unit>
+      <trans-unit id="tcUnitToObjSubsumption">
+        <source>This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</source>
+        <target state="new">This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</target>
+        <note />
+      </trans-unit>
       <trans-unit id="tcUsingInterfaceWithStaticAbstractMethodAsType">
         <source>'{0}' is normally used as a type constraint in generic code, e.g. \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\". See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3536\"' or '--nowarn:3536'.</source>
         <target state="translated">„{0}“ wird normalerweise als Typeinschränkung im generischen Code verwendet, z. B. \"'T when ISomeInterface&lt;'T&gt;\" oder \"let f (x: #ISomeInterface&lt;_&gt;)\". Anleitungen finden Sie unter https://aka.ms/fsharp-iwsams. Sie können diese Warnung deaktivieren, indem Sie „#nowarn \"3536\"“ or „--nowarn:3536“ verwenden.</target>
diff --git a/src/Compiler/xlf/FSComp.txt.es.xlf b/src/Compiler/xlf/FSComp.txt.es.xlf
index 5a84160317..9f042384d3 100644
--- a/src/Compiler/xlf/FSComp.txt.es.xlf
+++ b/src/Compiler/xlf/FSComp.txt.es.xlf
@@ -627,6 +627,11 @@
         <target state="new">Support ValueOption as valid type for optional member parameters</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureSupportWarnWhenUnitPassedToObjArg">
+        <source>Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</source>
+        <target state="new">Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureUnionIsPropertiesVisible">
         <source>Union case test properties</source>
         <target state="translated">Propiedades de prueba de caso de unión</target>
@@ -1712,6 +1717,11 @@
         <target state="translated">Tipo de función inesperado en la definición de campo de caso de unión. Si pretende que el campo sea una función, considere la posibilidad de encapsular la firma de función con paréntesis, por ejemplo, Case of a -&gt; b en | Case of (a -&gt; b).</target>
         <note />
       </trans-unit>
+      <trans-unit id="tcUnitToObjSubsumption">
+        <source>This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</source>
+        <target state="new">This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</target>
+        <note />
+      </trans-unit>
       <trans-unit id="tcUsingInterfaceWithStaticAbstractMethodAsType">
         <source>'{0}' is normally used as a type constraint in generic code, e.g. \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\". See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3536\"' or '--nowarn:3536'.</source>
         <target state="translated">'{0}' se usa normalmente como restricción de tipo en código genérico; por ejemplo, \"'T when ISomeInterface&lt;'T&gt;\" o \"let f (x: #ISomeInterface&lt;_&gt;)\". Consulte https://aka.ms/fsharp-iwsams para obtener instrucciones. Puede deshabilitar esta advertencia con "#nowarn \"3536\"" o "--nowarn:3536".</target>
diff --git a/src/Compiler/xlf/FSComp.txt.fr.xlf b/src/Compiler/xlf/FSComp.txt.fr.xlf
index 3e98a55816..621286a8e2 100644
--- a/src/Compiler/xlf/FSComp.txt.fr.xlf
+++ b/src/Compiler/xlf/FSComp.txt.fr.xlf
@@ -627,6 +627,11 @@
         <target state="new">Support ValueOption as valid type for optional member parameters</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureSupportWarnWhenUnitPassedToObjArg">
+        <source>Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</source>
+        <target state="new">Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureUnionIsPropertiesVisible">
         <source>Union case test properties</source>
         <target state="translated">Propriétés du test de cas d’union</target>
@@ -1712,6 +1717,11 @@
         <target state="translated">Type de fonction inattendu dans la définition du champ de cas d'union. Si vous souhaitez que le champ soit une fonction, envisagez d'envelopper la signature de la fonction avec des parenthèses, par exemple. | Cas de a -&gt; b dans | Cas de (a -&gt; b).</target>
         <note />
       </trans-unit>
+      <trans-unit id="tcUnitToObjSubsumption">
+        <source>This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</source>
+        <target state="new">This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</target>
+        <note />
+      </trans-unit>
       <trans-unit id="tcUsingInterfaceWithStaticAbstractMethodAsType">
         <source>'{0}' is normally used as a type constraint in generic code, e.g. \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\". See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3536\"' or '--nowarn:3536'.</source>
         <target state="translated">'{0}' est généralement utilisée comme contrainte de type dans le code générique, par exemple \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\". Consultez https://aka.ms/fsharp-iwsams pour obtenir de l’aide. Vous pouvez désactiver cet avertissement à l’aide de '#nowarn \"3536\"' or '--nowarn:3536'.</target>
diff --git a/src/Compiler/xlf/FSComp.txt.it.xlf b/src/Compiler/xlf/FSComp.txt.it.xlf
index b982714a14..1d7bdf56aa 100644
--- a/src/Compiler/xlf/FSComp.txt.it.xlf
+++ b/src/Compiler/xlf/FSComp.txt.it.xlf
@@ -627,6 +627,11 @@
         <target state="new">Support ValueOption as valid type for optional member parameters</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureSupportWarnWhenUnitPassedToObjArg">
+        <source>Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</source>
+        <target state="new">Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureUnionIsPropertiesVisible">
         <source>Union case test properties</source>
         <target state="translated">Proprietà test case di unione</target>
@@ -1712,6 +1717,11 @@
         <target state="translated">Tipo di funzione imprevisto nella definizione del campo del caso di unione. Se desideri che il campo sia una funzione, considera di eseguire il wrapping della firma della funzione con le parentesi, ad esempio | Case of a -&gt; b into | Case of (a -&gt; b).</target>
         <note />
       </trans-unit>
+      <trans-unit id="tcUnitToObjSubsumption">
+        <source>This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</source>
+        <target state="new">This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</target>
+        <note />
+      </trans-unit>
       <trans-unit id="tcUsingInterfaceWithStaticAbstractMethodAsType">
         <source>'{0}' is normally used as a type constraint in generic code, e.g. \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\". See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3536\"' or '--nowarn:3536'.</source>
         <target state="translated">'{0}' viene in genere usato come vincolo di tipo nel codice generico, ad esempio \"'T when ISomeInterface&lt;'T&gt;\" o \"let f (x: #ISomeInterface&lt;_&gt;)\". Per indicazioni, vedere https://aka.ms/fsharp-iwsams. È possibile disabilitare questo avviso usando '#nowarn \"3536\"' o '--nowarn:3536'.</target>
diff --git a/src/Compiler/xlf/FSComp.txt.ja.xlf b/src/Compiler/xlf/FSComp.txt.ja.xlf
index 3a23712a96..c928a51da3 100644
--- a/src/Compiler/xlf/FSComp.txt.ja.xlf
+++ b/src/Compiler/xlf/FSComp.txt.ja.xlf
@@ -627,6 +627,11 @@
         <target state="new">Support ValueOption as valid type for optional member parameters</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureSupportWarnWhenUnitPassedToObjArg">
+        <source>Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</source>
+        <target state="new">Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureUnionIsPropertiesVisible">
         <source>Union case test properties</source>
         <target state="translated">ユニオン ケースのテスト プロパティ</target>
@@ -1712,6 +1717,11 @@
         <target state="translated">共用体ケースのフィールド定義で予期しない関数型が発生しました。フィールドを関数にする場合は、関数シグネチャを parens でラップすることを検討してください、例えば、 | Case of a -&gt; b into | Case of (a -&gt; b)。</target>
         <note />
       </trans-unit>
+      <trans-unit id="tcUnitToObjSubsumption">
+        <source>This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</source>
+        <target state="new">This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</target>
+        <note />
+      </trans-unit>
       <trans-unit id="tcUsingInterfaceWithStaticAbstractMethodAsType">
         <source>'{0}' is normally used as a type constraint in generic code, e.g. \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\". See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3536\"' or '--nowarn:3536'.</source>
         <target state="translated">'{0}' は通常、ジェネリック コードの型制約として使用されます (例: \"'T when ISomeInterface&lt;'T&gt;\" または \"let f (x: #ISomeInterface&lt;_&gt;)\")。ガイダンスについては https://aka.ms/fsharp-iwsams を参照してください。この警告は、'#nowarn \"3536\"' または '--nowarn:3536' を使用して無効にできます。</target>
diff --git a/src/Compiler/xlf/FSComp.txt.ko.xlf b/src/Compiler/xlf/FSComp.txt.ko.xlf
index 41ad2ce40a..aaff373fcd 100644
--- a/src/Compiler/xlf/FSComp.txt.ko.xlf
+++ b/src/Compiler/xlf/FSComp.txt.ko.xlf
@@ -627,6 +627,11 @@
         <target state="new">Support ValueOption as valid type for optional member parameters</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureSupportWarnWhenUnitPassedToObjArg">
+        <source>Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</source>
+        <target state="new">Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureUnionIsPropertiesVisible">
         <source>Union case test properties</source>
         <target state="translated">공용 구조체 사례 테스트 속성</target>
@@ -1712,6 +1717,11 @@
         <target state="translated">공용 구조체 사례 필드 정의에 예기치 않은 함수 형식이 있습니다. 필드가 함수가 되도록 하려면 함수 시그니처를 구문 분석으로 래핑하는 것이 좋습니다(예: | Case of a -&gt; b into | Case of (a -&gt; b))</target>
         <note />
       </trans-unit>
+      <trans-unit id="tcUnitToObjSubsumption">
+        <source>This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</source>
+        <target state="new">This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</target>
+        <note />
+      </trans-unit>
       <trans-unit id="tcUsingInterfaceWithStaticAbstractMethodAsType">
         <source>'{0}' is normally used as a type constraint in generic code, e.g. \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\". See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3536\"' or '--nowarn:3536'.</source>
         <target state="translated">'{0}'은(는) 일반적으로 제네릭 코드에서 형식 제약 조건으로 사용됩니다(예: \"'T when ISomeInterface&lt;'T&gt;\" 또는 \"let f (x: #ISomeInterface&lt;_&gt;)\"). 지침은 https://aka.ms/fsharp-iwsams를 참조하세요. '#nowarn \"3536\"' 또는 '--nowarn:3536'을 사용하여 이 경고를 비활성화할 수 있습니다.</target>
diff --git a/src/Compiler/xlf/FSComp.txt.pl.xlf b/src/Compiler/xlf/FSComp.txt.pl.xlf
index f7cf199c4f..f886e72e8f 100644
--- a/src/Compiler/xlf/FSComp.txt.pl.xlf
+++ b/src/Compiler/xlf/FSComp.txt.pl.xlf
@@ -627,6 +627,11 @@
         <target state="new">Support ValueOption as valid type for optional member parameters</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureSupportWarnWhenUnitPassedToObjArg">
+        <source>Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</source>
+        <target state="new">Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureUnionIsPropertiesVisible">
         <source>Union case test properties</source>
         <target state="translated">Właściwości testowe przypadku unii</target>
@@ -1712,6 +1717,11 @@
         <target state="translated">Nieoczekiwany typ funkcji w definicji pola przypadku unii. Jeśli zamierzasz, aby pole było funkcją, rozważ ujęcie podpisu funkcji w nawiasy, np. | Przypadek a -&gt; b | Przypadek (a -&gt; b).</target>
         <note />
       </trans-unit>
+      <trans-unit id="tcUnitToObjSubsumption">
+        <source>This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</source>
+        <target state="new">This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</target>
+        <note />
+      </trans-unit>
       <trans-unit id="tcUsingInterfaceWithStaticAbstractMethodAsType">
         <source>'{0}' is normally used as a type constraint in generic code, e.g. \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\". See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3536\"' or '--nowarn:3536'.</source>
         <target state="translated">Element „{0}” jest zwykle używany jako ograniczenie typu w kodzie ogólnym, np. \"" T, gdy ISomeInterface&lt;' T&gt;\" lub \"let f (x: #ISomeInterface&lt;_&gt;)\". Aby uzyskać wskazówki, zobacz https://aka.ms/fsharp-iwsams. To ostrzeżenie można wyłączyć, używając polecenia „nowarn \"3536\"" lub "--nowarn:3536”.</target>
diff --git a/src/Compiler/xlf/FSComp.txt.pt-BR.xlf b/src/Compiler/xlf/FSComp.txt.pt-BR.xlf
index 4813c27846..6908257467 100644
--- a/src/Compiler/xlf/FSComp.txt.pt-BR.xlf
+++ b/src/Compiler/xlf/FSComp.txt.pt-BR.xlf
@@ -627,6 +627,11 @@
         <target state="new">Support ValueOption as valid type for optional member parameters</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureSupportWarnWhenUnitPassedToObjArg">
+        <source>Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</source>
+        <target state="new">Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureUnionIsPropertiesVisible">
         <source>Union case test properties</source>
         <target state="translated">Propriedades de teste de caso de união</target>
@@ -1712,6 +1717,11 @@
         <target state="translated">Tipo de função inesperado na definição de campo de caso de união. Se você pretende que o campo seja uma função, considere encapsular a assinatura de função com parênteses, por exemplo, | Caso de a -&gt; b em | Caso de (a -&gt; b).</target>
         <note />
       </trans-unit>
+      <trans-unit id="tcUnitToObjSubsumption">
+        <source>This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</source>
+        <target state="new">This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</target>
+        <note />
+      </trans-unit>
       <trans-unit id="tcUsingInterfaceWithStaticAbstractMethodAsType">
         <source>'{0}' is normally used as a type constraint in generic code, e.g. \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\". See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3536\"' or '--nowarn:3536'.</source>
         <target state="translated">'{0}' normalmente é usado como uma restrição de tipo em código genérico, por exemplo, \"'T when ISomeInterface&lt;'T&gt;\" ou \"let f (x: #ISomeInterface&lt;_&gt;)\". Confira https://aka.ms/fsharp-iwsams para obter as diretrizes. Você pode desabilitar este aviso usando '#nowarn \"3536\"' ou '--nowarn:3536'.</target>
diff --git a/src/Compiler/xlf/FSComp.txt.ru.xlf b/src/Compiler/xlf/FSComp.txt.ru.xlf
index b1aad1198d..df0d5f9770 100644
--- a/src/Compiler/xlf/FSComp.txt.ru.xlf
+++ b/src/Compiler/xlf/FSComp.txt.ru.xlf
@@ -627,6 +627,11 @@
         <target state="new">Support ValueOption as valid type for optional member parameters</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureSupportWarnWhenUnitPassedToObjArg">
+        <source>Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</source>
+        <target state="new">Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureUnionIsPropertiesVisible">
         <source>Union case test properties</source>
         <target state="translated">Свойства теста союзного случая</target>
@@ -1712,6 +1717,11 @@
         <target state="translated">Неожиданный тип функции в определении поля регистра объединения. Если предполагается, что поле будет представлять собой функцию, следует обернуть сигнатуру функции родительными падежами, например, | Case of a -&gt; b в | Case of (a -&gt; b).</target>
         <note />
       </trans-unit>
+      <trans-unit id="tcUnitToObjSubsumption">
+        <source>This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</source>
+        <target state="new">This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</target>
+        <note />
+      </trans-unit>
       <trans-unit id="tcUsingInterfaceWithStaticAbstractMethodAsType">
         <source>'{0}' is normally used as a type constraint in generic code, e.g. \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\". See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3536\"' or '--nowarn:3536'.</source>
         <target state="translated">"{0}" обычно используется в качестве ограничения типа в универсальном коде, например \"'T when ISomeInterface&lt;"T&gt;\" или \"let f (x: #ISomeInterface&lt;_&gt;)\". См. руководство на https://aka.ms/fsharp-iwsams. Это предупреждение можно отключить с помощью "#nowarn \"3536\"" или "--nowarn:3536".</target>
diff --git a/src/Compiler/xlf/FSComp.txt.tr.xlf b/src/Compiler/xlf/FSComp.txt.tr.xlf
index 050ccf6ea9..7756859313 100644
--- a/src/Compiler/xlf/FSComp.txt.tr.xlf
+++ b/src/Compiler/xlf/FSComp.txt.tr.xlf
@@ -627,6 +627,11 @@
         <target state="new">Support ValueOption as valid type for optional member parameters</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureSupportWarnWhenUnitPassedToObjArg">
+        <source>Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</source>
+        <target state="new">Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureUnionIsPropertiesVisible">
         <source>Union case test properties</source>
         <target state="translated">Birleşim durumu test özellikleri</target>
@@ -1712,6 +1717,11 @@
         <target state="translated">Birleşim durumu alan tanımında beklenmeyen işlev türü. Alanın bir işlev olmasını amaçlıyorsanız işlev imzasını parens ile sarmalamayı göz önünde bulundurun. Ör. | (a -&gt; b) durumunda | a -&gt; b durumu.</target>
         <note />
       </trans-unit>
+      <trans-unit id="tcUnitToObjSubsumption">
+        <source>This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</source>
+        <target state="new">This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</target>
+        <note />
+      </trans-unit>
       <trans-unit id="tcUsingInterfaceWithStaticAbstractMethodAsType">
         <source>'{0}' is normally used as a type constraint in generic code, e.g. \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\". See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3536\"' or '--nowarn:3536'.</source>
         <target state="translated">'{0}' normalde genel kodda tür kısıtlaması olarak kullanılır, ör. \"'T when ISomeInterface&lt;'T&gt;\" veya \"let f (x: #ISomeInterface&lt;_&gt;)\". Rehber için bkz. https://aka.ms/fsharp-iwsams. '#nowarn \"3536\"' veya '--nowarn:3536' kullanarak bu uyarıyı devre dışı bırakabilirsiniz.</target>
diff --git a/src/Compiler/xlf/FSComp.txt.zh-Hans.xlf b/src/Compiler/xlf/FSComp.txt.zh-Hans.xlf
index 085668fbc0..59532dc06d 100644
--- a/src/Compiler/xlf/FSComp.txt.zh-Hans.xlf
+++ b/src/Compiler/xlf/FSComp.txt.zh-Hans.xlf
@@ -627,6 +627,11 @@
         <target state="new">Support ValueOption as valid type for optional member parameters</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureSupportWarnWhenUnitPassedToObjArg">
+        <source>Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</source>
+        <target state="new">Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureUnionIsPropertiesVisible">
         <source>Union case test properties</source>
         <target state="translated">联合用例测试属性</target>
@@ -1712,6 +1717,11 @@
         <target state="translated">联合用例字段定义中出现意外的函数类型。如果打算将字段作为函数，请考虑使用括号括起函数签名，例如 | Case of a -&gt; b into | Case of (a -&gt; b)。</target>
         <note />
       </trans-unit>
+      <trans-unit id="tcUnitToObjSubsumption">
+        <source>This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</source>
+        <target state="new">This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</target>
+        <note />
+      </trans-unit>
       <trans-unit id="tcUsingInterfaceWithStaticAbstractMethodAsType">
         <source>'{0}' is normally used as a type constraint in generic code, e.g. \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\". See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3536\"' or '--nowarn:3536'.</source>
         <target state="translated">"{0}" 通常用作泛型代码中的类型约束，例如 \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\"。有关指南，请参阅 https://aka.ms/fsharp-iwsams。可以使用 '#nowarn \"3536\"' 或 '--nowarn:3536' 禁用此警告。</target>
diff --git a/src/Compiler/xlf/FSComp.txt.zh-Hant.xlf b/src/Compiler/xlf/FSComp.txt.zh-Hant.xlf
index 48f36e8611..2274eb373c 100644
--- a/src/Compiler/xlf/FSComp.txt.zh-Hant.xlf
+++ b/src/Compiler/xlf/FSComp.txt.zh-Hant.xlf
@@ -627,6 +627,11 @@
         <target state="new">Support ValueOption as valid type for optional member parameters</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureSupportWarnWhenUnitPassedToObjArg">
+        <source>Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</source>
+        <target state="new">Warn when unit is passed to a member accepting `obj` argument, e.g. `Method(o:obj)` will warn if called via `Method()`.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureUnionIsPropertiesVisible">
         <source>Union case test properties</source>
         <target state="translated">聯集案例測試屬性</target>
@@ -1712,6 +1717,11 @@
         <target state="translated">聯集案例欄位定義中有未預期的函式類型。如果您想要讓欄位成為函式，請考慮使用括弧括住函式簽章，例如將 | Case of a -&gt; b 變更為 | Case of (a -&gt; b)。</target>
         <note />
       </trans-unit>
+      <trans-unit id="tcUnitToObjSubsumption">
+        <source>This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</source>
+        <target state="new">This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn \"3397\".</target>
+        <note />
+      </trans-unit>
       <trans-unit id="tcUsingInterfaceWithStaticAbstractMethodAsType">
         <source>'{0}' is normally used as a type constraint in generic code, e.g. \"'T when ISomeInterface&lt;'T&gt;\" or \"let f (x: #ISomeInterface&lt;_&gt;)\". See https://aka.ms/fsharp-iwsams for guidance. You can disable this warning by using '#nowarn \"3536\"' or '--nowarn:3536'.</source>
         <target state="translated">'{0}' 通常做為一般程式碼中的類型限制式，例如 \"'T when ISomeInterface&lt;'T&gt;\" 或 \"let f (x: #ISomeInterface&lt;_&gt;)\"。請參閱 https://aka.ms/fsharp-iwsams 以尋求指引。您可以使用 '#nowarn \"3536\"' 或 '--nowarn:3536' 來停用此警告。</target>
diff --git a/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OptionalDefaultParamArgs/OptionalDefaultParamArgs.fs b/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OptionalDefaultParamArgs/OptionalDefaultParamArgs.fs
index ae25dc7219..8ba293dce2 100644
--- a/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OptionalDefaultParamArgs/OptionalDefaultParamArgs.fs
+++ b/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OptionalDefaultParamArgs/OptionalDefaultParamArgs.fs
@@ -73,12 +73,25 @@ module MemberDefinitions_OptionalDefaultParamArgs =
     [<Theory; Directory(__SOURCE_DIRECTORY__, Includes=[|"W_WrongDefaultType.fs"|])>]
     let ``W_WrongDefaultType_fs`` compilation =
         compilation
+        |> withLangVersionPreview
         |> verifyCompile
         |> shouldFail
-        |> withDiagnostics [
-            (Warning 3211, Line 10, Col 62, Line 10, Col 63, "The default value does not have the same type as the argument. The DefaultParameterValue attribute and any Optional attribute will be ignored. Note: 'null' needs to be annotated with the correct type, e.g. 'DefaultParameterValue(null:obj)'.")
-        ]
-
+        |> withDiagnostics 
+                      [ Warning 3211, Line 10, Col 62, Line 10, Col 63, "The default value does not have the same type as the argument. The DefaultParameterValue attribute and any Optional attribute will be ignored. Note: 'null' needs to be annotated with the correct type, e.g. 'DefaultParameterValue(null:obj)'."
+                        Error 1, Line 13, Col 25, Line 13, Col 27, "This expression was expected to have type
+                        'string' 
+                        but here has type
+                        'unit' "]
+
+    [<Theory; Directory(__SOURCE_DIRECTORY__, Includes=[|"W_WrongDefaultObj.fs"|])>]
+    let ``W_WrongDefaultObjType_fs`` compilation =
+        compilation
+        |> withLangVersionPreview
+        |> verifyCompile
+        |> shouldFail
+        |> withDiagnostics 
+            [ Warning 3211, Line 10, Col 62, Line 10, Col 67, "The default value does not have the same type as the argument. The DefaultParameterValue attribute and any Optional attribute will be ignored. Note: 'null' needs to be annotated with the correct type, e.g. 'DefaultParameterValue(null:obj)'."
+              Warning 3397, Line 12, Col 19, Line 12, Col 21, """This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime. This warning may be disabled using '#nowarn "3397"."""]
 
 
 
diff --git a/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OptionalDefaultParamArgs/W_WrongDefaultObj.fs b/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OptionalDefaultParamArgs/W_WrongDefaultObj.fs
new file mode 100644
index 0000000000..b0e95c0974
--- /dev/null
+++ b/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OptionalDefaultParamArgs/W_WrongDefaultObj.fs
@@ -0,0 +1,13 @@
+// #Conformance #DeclarationElements #MemberDefinitions #OptionalDefaultParameterValueArguments
+
+
+//<Expects>The DefaultParameterValue attribute and any Optional attribute will be ignored.</Expects>
+//<Expects>Note: 'null' needs to be annotated with the correct type, e.g. 'DefaultParameterValue(null:obj)'.</Expects>
+
+open System.Runtime.InteropServices
+
+type Class() =
+    static member WrongType([<Optional;DefaultParameterValue("xxx")>]b:obj) = printfn "%A" b
+
+do Class.WrongType()
+do Class.WrongType("")
\ No newline at end of file
diff --git a/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OptionalDefaultParamArgs/W_WrongDefaultType.fs b/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OptionalDefaultParamArgs/W_WrongDefaultType.fs
index 89b002a923..8844f1bc8b 100644
--- a/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OptionalDefaultParamArgs/W_WrongDefaultType.fs
+++ b/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OptionalDefaultParamArgs/W_WrongDefaultType.fs
@@ -10,5 +10,6 @@ type Class() =
     static member WrongType([<Optional;DefaultParameterValue(1)>]b:string) = b
 
 let r = Class.WrongType("123")
+let r2 = Class.WrongType()
 
 do ()
\ No newline at end of file
diff --git a/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/AsyncModule.fs b/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/AsyncModule.fs
index d5bf29374d..b4e54ab6f4 100644
--- a/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/AsyncModule.fs
+++ b/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/AsyncModule.fs
@@ -12,6 +12,9 @@ open FSharp.Core.UnitTests.LibraryTestFx
 open Xunit
 open FsCheck
 
+#nowarn "3397" // This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime.
+// Why warned - the tests here are actually trying to assert that Async<unit> still works.
+
 module Utils =
     let internal memoizeAsync f =
         let cache = System.Collections.Concurrent.ConcurrentDictionary<'a, System.Threading.Tasks.Task<'b>>()
diff --git a/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/LazyType.fs b/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/LazyType.fs
index 30643db549..9bc8ec3e7d 100644
--- a/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/LazyType.fs
+++ b/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/LazyType.fs
@@ -8,6 +8,9 @@ open Xunit
 open Microsoft.FSharp.Collections
 open FSharp.Core.UnitTests.LibraryTestFx
 
+#nowarn "3397" // This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime.
+// Why warned - the tests here are actually trying to test that when this happens (unit passed), it indeed results in a null
+
 type LazyType() =
    
     [<Fact>]
diff --git a/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Reflection/FSharpReflection.fs b/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Reflection/FSharpReflection.fs
index 56fd8a0a1b..fabb893620 100644
--- a/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Reflection/FSharpReflection.fs
+++ b/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Reflection/FSharpReflection.fs
@@ -12,6 +12,9 @@ open Xunit
 
 open Microsoft.FSharp.Reflection
 
+#nowarn "3397" // This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime.
+// Why warned - the tests here are testing also how APIs react to unit being passed to it.
+
 (*
 [Test Strategy]
 Make sure each method works on:
diff --git a/tests/FSharp.Core.UnitTests/FSharp.Core/OperatorsModule1.fs b/tests/FSharp.Core.UnitTests/FSharp.Core/OperatorsModule1.fs
index 727ed166b8..ec620ebc22 100644
--- a/tests/FSharp.Core.UnitTests/FSharp.Core/OperatorsModule1.fs
+++ b/tests/FSharp.Core.UnitTests/FSharp.Core/OperatorsModule1.fs
@@ -13,6 +13,9 @@ open System
 open FSharp.Core.UnitTests.LibraryTestFx
 open Xunit
 
+#nowarn "3397" // This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime.
+// Why warned - the tests here are actually trying to test that when this happens (unit passed), it indeed results in a null
+
 type OperatorsModule1() =
 
     [<Fact>]
diff --git a/tests/FSharp.Core.UnitTests/FSharp.Core/OperatorsModule2.fs b/tests/FSharp.Core.UnitTests/FSharp.Core/OperatorsModule2.fs
index f2dc93d9ee..3e0ea65274 100644
--- a/tests/FSharp.Core.UnitTests/FSharp.Core/OperatorsModule2.fs
+++ b/tests/FSharp.Core.UnitTests/FSharp.Core/OperatorsModule2.fs
@@ -19,6 +19,8 @@ open FSharp.Core.UnitTests.LibraryTestFx
 open Xunit
 
 #nowarn "3370"
+#nowarn "3397" // This expression uses 'unit' for an 'obj'-typed argument. This will lead to passing 'null' at runtime.
+// Why warned - the tests here are actually trying to test that when this happens (unit passed), it indeed results in a null
 
 /// If this type compiles without error it is correct
 /// Wrong if you see: FS0670 This code is not sufficiently generic. The type variable ^T could not be generalized because it would escape its scope.