--- a/dom/media/webaudio/MediaStreamAudioSourceNode.cpp
+++ b/dom/media/webaudio/MediaStreamAudioSourceNode.cpp
@@ -193,17 +193,19 @@ MediaStreamAudioSourceNode::PrincipalCha
 }
 
 size_t
 MediaStreamAudioSourceNode::SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const
 {
   // Future:
   // - mInputStream
   size_t amount = AudioNode::SizeOfExcludingThis(aMallocSizeOf);
-  amount += mInputPort->SizeOfIncludingThis(aMallocSizeOf);
+  if (mInputPort) {
+    amount += mInputPort->SizeOfIncludingThis(aMallocSizeOf);
+  }
   return amount;
 }
 
 size_t
 MediaStreamAudioSourceNode::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
 {
   return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf);
 }