diff --git a/reactos/ntoskrnl/ex/callback.c b/reactos/ntoskrnl/ex/callback.c
index 45c85af064a..cb9ed938748 100644
--- a/reactos/ntoskrnl/ex/callback.c
+++ b/reactos/ntoskrnl/ex/callback.c
@@ -21,6 +21,7 @@ VOID ExCallUserCallBack(PVOID fn)
  * FUNCTION: Transfer control to a user callback
  */
 {
+   UNIMPLEMENTED;
 }
 
 NTSTATUS STDCALL NtCallbackReturn(VOID)