--- a/dom/media/platforms/wmf/WMFMediaDataDecoder.cpp
+++ b/dom/media/platforms/wmf/WMFMediaDataDecoder.cpp
@@ -236,12 +236,14 @@ WMFMediaDataDecoder::SetSeekThreshold(co
 
   RefPtr<WMFMediaDataDecoder> self = this;
   nsCOMPtr<nsIRunnable> runnable =
     NS_NewRunnableFunction("WMFMediaDataDecoder::SetSeekThreshold",
                            [self, aTime]() {
     media::TimeUnit threshold = aTime;
     self->mMFTManager->SetSeekThreshold(threshold);
   });
-  mTaskQueue->Dispatch(runnable.forget());
+  nsresult rv = mTaskQueue->Dispatch(runnable.forget());
+  MOZ_DIAGNOSTIC_ASSERT(NS_SUCCEEDED(rv));
+  Unused << rv;
 }
 
 } // namespace mozilla