--- a/dom/media/platforms/wmf/MFTDecoder.cpp
+++ b/dom/media/platforms/wmf/MFTDecoder.cpp
@@ -212,16 +212,23 @@ MFTDecoder::Output(RefPtr<IMFSample>* aO
   if (output.pEvents) {
     // We must release this, as per the IMFTransform::ProcessOutput()
     // MSDN documentation.
     output.pEvents->Release();
     output.pEvents = nullptr;
   }
 
   if (hr == MF_E_TRANSFORM_STREAM_CHANGE) {
+    if (status == MFT_PROCESS_OUTPUT_STATUS_NEW_STREAMS) {
+      DWORD inputCount, outputCount;
+      mDecoder->GetStreamCount(&inputCount, &outputCount);
+      if (inputCount != outputCount) {
+        MOZ_CRASH("Got unexpected new output stream.");
+      }
+    }
     // Type change, probably geometric aperture change.
     // Reconfigure decoder output type, so that GetOutputMediaType()
     // returns the new type, and return the error code to caller.
     // This is an expected failure, so don't warn on encountering it.
     hr = SetDecoderOutputType(nullptr, nullptr);
     NS_ENSURE_TRUE(SUCCEEDED(hr), hr);
     // Return the error, so that the caller knows to retry.
     return MF_E_TRANSFORM_STREAM_CHANGE;