--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -2294,17 +2294,20 @@ void MediaDecoderStateMachine::Recompute
       // any other duration sources), but the duration isn't ready yet.
       return;
     }
     // We don't fire duration changed for this case because it should have
     // already been fired on the main thread when the explicit duration was set.
     duration = TimeUnit::FromSeconds(d);
   } else if (mEstimatedDuration.Ref().isSome()) {
     duration = mEstimatedDuration.Ref().ref();
-  } else if (Info().mMetadataDuration.isSome()) {
+  } else if (mInfo.isSome() && Info().mMetadataDuration.isSome()) {
+    // We need to check mInfo.isSome() because that this method might be invoked
+    // while mObservedDuration is changed which might before the metadata been
+    // read.
     duration = Info().mMetadataDuration.ref();
   } else {
     return;
   }
 
   // Only adjust the duration when an explicit duration isn't set (MSE).
   // The duration is always exactly known with MSE and there's no need to adjust
   // it based on what may have been seen in the past; in particular as this data