--- a/dom/media/MediaData.h
+++ b/dom/media/MediaData.h
@@ -235,18 +235,22 @@ private:
     }
 
     // Find alignment address.
     const uintptr_t alignmask = AlignmentOffset();
     Type* newData = reinterpret_cast<Type*>(
       (reinterpret_cast<uintptr_t>(newBuffer.get()) + alignmask) & ~alignmask);
     MOZ_ASSERT(uintptr_t(newData) % (AlignmentOffset()+1) == 0);
 
+    MOZ_ASSERT(!mLength || mData);
+
     PodZero(newData + mLength, aLength - mLength);
-    PodCopy(newData, mData, mLength);
+    if (mLength) {
+      PodCopy(newData, mData, mLength);
+    }
 
     mBuffer = Move(newBuffer);
     mCapacity = sizeNeeded.value();
     mData = newData;
 
     return true;
   }
   Type* mData;