diff --git a/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md b/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
index 54236b3864c..2e58a160d38 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
@@ -95,6 +95,7 @@
 * Fix Debug-mode compilation when mixing resumable and standard computation expressions. ([Issue #19625](https://github.com/dotnet/fsharp/issues/19625), [PR #19630](https://github.com/dotnet/fsharp/pull/19630))
 * IlxGen: fix missing CompilationMapping attribute for generic values ([PR #19643](https://github.com/dotnet/fsharp/pull/19643))
 * Fix internal error `FS0192: encodeCustomAttrElemType` when using arrays of user-defined types as custom attribute arguments. Empty arrays (e.g. `[<DefaultValue([||] : A[])>]`) now compile successfully; non-empty arrays of unencodable types report a proper diagnostic (FS3887) instead of an internal error. ([Issue #12796](https://github.com/dotnet/fsharp/issues/12796), [PR #19472](https://github.com/dotnet/fsharp/pull/19472))
+* Fix a compiler crash when calling an inherited base method on an external generic IL type, such as `base.OnDetaching()` on `Microsoft.Xaml.Interactivity.Behavior<'T>`. The IL base-call check now only applies the abstract-base guard when the method actually exists on the raw metadata type instead of crashing while trying to resolve an inherited method. ([Issue #20264](https://github.com/dotnet/fsharp/issues/20264))
 * Fix internal compiler error in `use` bindings when a C#-style `Dispose` extension method is in scope alongside `IDisposable.Dispose`. ([Issue #19552](https://github.com/dotnet/fsharp/issues/19552), [PR #19568](https://github.com/dotnet/fsharp/pull/19568))
 * Fix signature generation: single-case struct DU gets spurious bar causing FS0300. ([Issue #19597](https://github.com/dotnet/fsharp/issues/19597), [PR #19609](https://github.com/dotnet/fsharp/pull/19609))
 * Fix signature generation: backticked active pattern case names lose escaping. ([Issue #19592](https://github.com/dotnet/fsharp/issues/19592), [PR #19609](https://github.com/dotnet/fsharp/pull/19609))
diff --git a/src/Compiler/AbstractIL/il.fsi b/src/Compiler/AbstractIL/il.fsi
index aa2e7c69b2f..0f186dc5c09 100644
--- a/src/Compiler/AbstractIL/il.fsi
+++ b/src/Compiler/AbstractIL/il.fsi
@@ -1262,6 +1262,8 @@ type ILMethodDefs =
 
     member FindByName: string -> ILMethodDef list
 
+    member internal FindByNameAndArity: string * int -> ILMethodDef list
+
     member TryFindInstanceByNameAndCallingSignature: string * ILCallingSignature -> ILMethodDef option
 
 /// Field definitions.
diff --git a/src/Compiler/Checking/PostInferenceChecks.fs b/src/Compiler/Checking/PostInferenceChecks.fs
index 234783d7f3e..f737b5f43b5 100644
--- a/src/Compiler/Checking/PostInferenceChecks.fs
+++ b/src/Compiler/Checking/PostInferenceChecks.fs
@@ -1366,15 +1366,17 @@ and CheckILBaseCall cenv env (ilMethRef, enclTypeInst, methInst, retTypes, tyarg
     // Disallow calls to abstract base methods on IL types.
     match tryTcrefOfAppTy g baseVal.Type with
     | ValueSome tcref when tcref.IsILTycon ->
-        try
-            let mdef =
-                match tcref.ILTyconInfo with
-                | TILObjectReprData(scoref, _, _) ->
-                    resolveILMethodRefWithRescope (rescopeILType scoref) tcref.ILTyconRawMetadata ilMethRef
-
-            if mdef.IsAbstract then
-                errorR(Error(FSComp.SR.tcCannotCallAbstractBaseMember(RichText.mkMethod mdef.Name), m))
-        with _ -> ()
+        match tcref.ILTyconInfo with
+        | TILObjectReprData(scoref, _, _) ->
+            if not (isNil (tcref.ILTyconRawMetadata.Methods.FindByNameAndArity(ilMethRef.Name, ilMethRef.ArgTypes.Length))) then
+                try
+                    let mdef =
+                        resolveILMethodRefWithRescope (rescopeILType scoref) tcref.ILTyconRawMetadata ilMethRef
+
+                    if mdef.IsAbstract then
+                        errorR(Error(FSComp.SR.tcCannotCallAbstractBaseMember(RichText.mkMethod mdef.Name), m))
+                with _ ->
+                    ()
     | _ -> ()
 
     CheckTypeInstNoByrefs cenv env m tyargs
diff --git a/tests/FSharp.Compiler.ComponentTests/Interop/SimpleInteropTests.fs b/tests/FSharp.Compiler.ComponentTests/Interop/SimpleInteropTests.fs
index 00a5781553f..e8b686f7fb8 100644
--- a/tests/FSharp.Compiler.ComponentTests/Interop/SimpleInteropTests.fs
+++ b/tests/FSharp.Compiler.ComponentTests/Interop/SimpleInteropTests.fs
@@ -223,3 +223,35 @@ let main _ =
         |> asExe
         |> compileExeAndRun
         |> shouldSucceed
+
+    [<Fact>]
+    let ``Issue 20264 - inherited generic base method call does not crash`` () =
+        let csLib =
+            CSharp
+                """
+namespace External
+{
+    public class BehaviorBase { public virtual void OnDetaching() { } }
+    public class Behavior<T> : BehaviorBase { }
+}
+                """
+            |> withName "ExternalBehavior"
+
+        let fsLib =
+            FSharp
+                """
+module TestIssue20264
+
+open External
+
+type MyBehavior() =
+    inherit Behavior<int>()
+
+    override _.OnDetaching() =
+        base.OnDetaching()
+                """
+            |> withReferences [ csLib ]
+
+        fsLib
+        |> compile
+        |> shouldSucceed