diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 3be17bfdaa6da..23a49e551757c 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -25891,6 +25891,10 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 if (!nonConstructorTypeInUnion) return type;
             }
 
+            if (assumeTrue && declaredType === unknownType && isEmptyAnonymousObjectType(type)) {
+                return targetType;
+            }
+
             return getNarrowedType(type, targetType, assumeTrue, /*checkDerived*/ true);
         }
 
diff --git a/tests/baselines/reference/inKeywordAndUnknown.errors.txt b/tests/baselines/reference/inKeywordAndUnknown.errors.txt
index 61c2cd2a2a364..41dde018c8913 100644
--- a/tests/baselines/reference/inKeywordAndUnknown.errors.txt
+++ b/tests/baselines/reference/inKeywordAndUnknown.errors.txt
@@ -20,4 +20,17 @@ tests/cases/compiler/inKeywordAndUnknown.ts(12,18): error TS2638: Type '{}' may
         }
         y;  // {}
     }
+    
+    
+    // repro #51007
+    function isHTMLTable(table: unknown): boolean {
+        return !!table && table instanceof Object && 'html' in table;
+    }
+    
+    function tryGetHtmlPropFromTable(table: unknown) {
+        if (!!table && table instanceof Object && 'html' in table && table.html instanceof Object) {
+            return table.html;
+        }
+        return undefined;
+    }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/inKeywordAndUnknown.js b/tests/baselines/reference/inKeywordAndUnknown.js
index 354bb8c4b1884..9a25c77e6268c 100644
--- a/tests/baselines/reference/inKeywordAndUnknown.js
+++ b/tests/baselines/reference/inKeywordAndUnknown.js
@@ -15,6 +15,19 @@ function f(x: {}, y: unknown) {
     }
     y;  // {}
 }
+
+
+// repro #51007
+function isHTMLTable(table: unknown): boolean {
+    return !!table && table instanceof Object && 'html' in table;
+}
+
+function tryGetHtmlPropFromTable(table: unknown) {
+    if (!!table && table instanceof Object && 'html' in table && table.html instanceof Object) {
+        return table.html;
+    }
+    return undefined;
+}
 
 
 //// [inKeywordAndUnknown.js]
@@ -34,3 +47,13 @@ function f(x, y) {
     }
     y; // {}
 }
+// repro #51007
+function isHTMLTable(table) {
+    return !!table && table instanceof Object && 'html' in table;
+}
+function tryGetHtmlPropFromTable(table) {
+    if (!!table && table instanceof Object && 'html' in table && table.html instanceof Object) {
+        return table.html;
+    }
+    return undefined;
+}
diff --git a/tests/baselines/reference/inKeywordAndUnknown.symbols b/tests/baselines/reference/inKeywordAndUnknown.symbols
index 145e92dc7f657..ddef5c201b058 100644
--- a/tests/baselines/reference/inKeywordAndUnknown.symbols
+++ b/tests/baselines/reference/inKeywordAndUnknown.symbols
@@ -31,3 +31,39 @@ function f(x: {}, y: unknown) {
 >y : Symbol(y, Decl(inKeywordAndUnknown.ts, 2, 17))
 }
 
+
+// repro #51007
+function isHTMLTable(table: unknown): boolean {
+>isHTMLTable : Symbol(isHTMLTable, Decl(inKeywordAndUnknown.ts, 15, 1))
+>table : Symbol(table, Decl(inKeywordAndUnknown.ts, 19, 21))
+
+    return !!table && table instanceof Object && 'html' in table;
+>table : Symbol(table, Decl(inKeywordAndUnknown.ts, 19, 21))
+>table : Symbol(table, Decl(inKeywordAndUnknown.ts, 19, 21))
+>Object : Symbol(Object, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>table : Symbol(table, Decl(inKeywordAndUnknown.ts, 19, 21))
+}
+
+function tryGetHtmlPropFromTable(table: unknown) {
+>tryGetHtmlPropFromTable : Symbol(tryGetHtmlPropFromTable, Decl(inKeywordAndUnknown.ts, 21, 1))
+>table : Symbol(table, Decl(inKeywordAndUnknown.ts, 23, 33))
+
+    if (!!table && table instanceof Object && 'html' in table && table.html instanceof Object) {
+>table : Symbol(table, Decl(inKeywordAndUnknown.ts, 23, 33))
+>table : Symbol(table, Decl(inKeywordAndUnknown.ts, 23, 33))
+>Object : Symbol(Object, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>table : Symbol(table, Decl(inKeywordAndUnknown.ts, 23, 33))
+>table.html : Symbol(html)
+>table : Symbol(table, Decl(inKeywordAndUnknown.ts, 23, 33))
+>html : Symbol(html)
+>Object : Symbol(Object, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+        return table.html;
+>table.html : Symbol(html)
+>table : Symbol(table, Decl(inKeywordAndUnknown.ts, 23, 33))
+>html : Symbol(html)
+    }
+    return undefined;
+>undefined : Symbol(undefined)
+}
+
diff --git a/tests/baselines/reference/inKeywordAndUnknown.types b/tests/baselines/reference/inKeywordAndUnknown.types
index 59a587598b23d..a19888f7a9a3d 100644
--- a/tests/baselines/reference/inKeywordAndUnknown.types
+++ b/tests/baselines/reference/inKeywordAndUnknown.types
@@ -40,3 +40,55 @@ function f(x: {}, y: unknown) {
 >y : Record<"a", unknown>
 }
 
+
+// repro #51007
+function isHTMLTable(table: unknown): boolean {
+>isHTMLTable : (table: unknown) => boolean
+>table : unknown
+
+    return !!table && table instanceof Object && 'html' in table;
+>!!table && table instanceof Object && 'html' in table : boolean
+>!!table && table instanceof Object : boolean
+>!!table : boolean
+>!table : boolean
+>table : unknown
+>table instanceof Object : boolean
+>table : {}
+>Object : ObjectConstructor
+>'html' in table : boolean
+>'html' : "html"
+>table : Object
+}
+
+function tryGetHtmlPropFromTable(table: unknown) {
+>tryGetHtmlPropFromTable : (table: unknown) => Object | undefined
+>table : unknown
+
+    if (!!table && table instanceof Object && 'html' in table && table.html instanceof Object) {
+>!!table && table instanceof Object && 'html' in table && table.html instanceof Object : boolean
+>!!table && table instanceof Object && 'html' in table : boolean
+>!!table && table instanceof Object : boolean
+>!!table : boolean
+>!table : boolean
+>table : unknown
+>table instanceof Object : boolean
+>table : {}
+>Object : ObjectConstructor
+>'html' in table : boolean
+>'html' : "html"
+>table : Object
+>table.html instanceof Object : boolean
+>table.html : unknown
+>table : Object & Record<"html", unknown>
+>html : unknown
+>Object : ObjectConstructor
+
+        return table.html;
+>table.html : Object
+>table : Object & Record<"html", unknown>
+>html : Object
+    }
+    return undefined;
+>undefined : undefined
+}
+
diff --git a/tests/cases/compiler/inKeywordAndUnknown.ts b/tests/cases/compiler/inKeywordAndUnknown.ts
index 745dfbc1f9af5..cf27e39736894 100644
--- a/tests/cases/compiler/inKeywordAndUnknown.ts
+++ b/tests/cases/compiler/inKeywordAndUnknown.ts
@@ -16,3 +16,16 @@ function f(x: {}, y: unknown) {
     }
     y;  // {}
 }
+
+
+// repro #51007
+function isHTMLTable(table: unknown): boolean {
+    return !!table && table instanceof Object && 'html' in table;
+}
+
+function tryGetHtmlPropFromTable(table: unknown) {
+    if (!!table && table instanceof Object && 'html' in table && table.html instanceof Object) {
+        return table.html;
+    }
+    return undefined;
+}