diff --git a/RELEASE_NOTES.md b/RELEASE_NOTES.md
index bb03368b4..3c7b3733e 100644
--- a/RELEASE_NOTES.md
+++ b/RELEASE_NOTES.md
@@ -1,5 +1,10 @@
 # Release Notes
 
+## 8.1.1 - Mar 14 2026
+
+- Fix `CombinedStream` to throw `NotSupportedException` (instead of `Exception`) for unsupported operations (write, seek, set length); fixes a typo in the length error message
+- Rename shadowed inner `isText` helper to `isMimeTypeText` for code clarity; add explanatory comment
+
 ## 8.1.0 - Mar 08 2026
 
 - Add schema.org microdata support to `HtmlProvider`: when an HTML document contains elements with `itemscope`/`itemtype`/`itemprop` attributes, the provider now generates a typed `Schemas` container (e.g. `doc.Schemas.Person`) with one strongly-typed property per `itemprop` name discovered in the sample (closes #611)
diff --git a/src/FSharp.Data.Http/Http.fs b/src/FSharp.Data.Http/Http.fs
index 8cc2f09c0..52c67d53c 100644
--- a/src/FSharp.Data.Http/Http.fs
+++ b/src/FSharp.Data.Http/Http.fs
@@ -1499,18 +1499,31 @@ module internal HttpHelpers =
         override x.Length =
             length
             |> Option.defaultWith (fun () ->
-                failwith "One or more of the encompassed streams are not seekable and the length cannot be determine")
+                raise (
+                    NotSupportedException(
+                        "One or more of the encompassed streams are not seekable and the length cannot be determined"
+                    )
+                ))
 
         override x.Position
             with get () = v
-            and set (_) = failwith "no position setting"
+            and set (_) = raise (NotSupportedException("CombinedStream does not support seeking"))
 
         override x.Flush() = ()
         override x.CanTimeout = false
-        override x.Seek(_, _) = failwith "no seeking"
-        override x.SetLength(_) = failwith "no setting length"
-        override x.Write(_, _, _) = failwith "no writing"
-        override x.WriteByte(_) = failwith "seriously, no writing"
+
+        override x.Seek(_, _) =
+            raise (NotSupportedException("CombinedStream does not support seeking"))
+
+        override x.SetLength(_) =
+            raise (NotSupportedException("CombinedStream does not support setting length"))
+
+        override x.Write(_, _, _) =
+            raise (NotSupportedException("CombinedStream is read-only"))
+
+        override x.WriteByte(_) =
+            raise (NotSupportedException("CombinedStream is read-only"))
+
         override x.Read(buffer, offset, count) = readFromStream buffer offset count
 
         interface IDisposable with
@@ -1804,7 +1817,7 @@ module internal HttpHelpers =
         async {
 
             let isText (mimeType: string) =
-                let isText (mimeType: string) =
+                let isMimeTypeText (mimeType: string) =
                     let mimeType = mimeType.Trim()
 
                     mimeType.StartsWith("text/", StringComparison.Ordinal)
@@ -1819,8 +1832,9 @@ module internal HttpHelpers =
                     || mimeType.StartsWith("application/", StringComparison.Ordinal)
                        && mimeType.EndsWith("+json", StringComparison.Ordinal)
 
+                // Split on ';' to ignore MIME type parameters (e.g. "text/html; charset=utf-8")
                 mimeType.Split([| ';' |], StringSplitOptions.RemoveEmptyEntries)
-                |> Array.exists isText
+                |> Array.exists isMimeTypeText
 
             let! memoryStream = asyncRead stream
 
diff --git a/tests/FSharp.Data.Core.Tests/Http.fs b/tests/FSharp.Data.Core.Tests/Http.fs
index 61cb29b41..a8524f121 100644
--- a/tests/FSharp.Data.Core.Tests/Http.fs
+++ b/tests/FSharp.Data.Core.Tests/Http.fs
@@ -366,7 +366,7 @@ let ``CombinedStream can seek with Some length`` () =
 [<Test>]
 let ``CombinedStream length throws with None length`` () =
     use combinedStream = new HttpHelpers.CombinedStream(None, [])
-    (fun () -> combinedStream.Length |> ignore) |> should throw typeof<Exception>
+    (fun () -> combinedStream.Length |> ignore) |> should throw typeof<NotSupportedException>
     
 [<Test>]
 let ``CombinedStream cannot seek with None length`` () =
@@ -383,7 +383,7 @@ let ``Non-seekable streams create non-seekable CombinedStream`` () =
     use nonSeekms = new nonSeekableStream(Array.zeroCreate 10)
     let multiparts = [MultipartItem("","", nonSeekms)]
     let combinedStream = HttpHelpers.writeMultipart "-" multiparts Encoding.UTF8
-    (fun () -> combinedStream.Length |> ignore) |> should throw typeof<Exception>
+    (fun () -> combinedStream.Length |> ignore) |> should throw typeof<NotSupportedException>
     combinedStream.CanSeek |> should equal false
     
 [<Test>]