--- a/dom/media/MediaCache.cpp
+++ b/dom/media/MediaCache.cpp
@@ -2487,17 +2487,17 @@ MediaCacheStream::Read(char* aBuffer, ui
         bytes = std::min(bytes, int64_t(INT32_MAX));
         MOZ_ASSERT(bytes >= 0 && bytes <= aCount, "Bytes out of range.");
         memcpy(aBuffer,
           streamWithPartialBlock->mPartialBlockBuffer.get() + offsetInStreamBlock, bytes);
         if (mCurrentMode == MODE_METADATA) {
           streamWithPartialBlock->mMetadataInPartialBlockBuffer = true;
         }
         streamOffset += bytes;
-        count = bytes;
+        count += bytes;
         break;
       }
 
       // No data has been read yet, so block
       mon.Wait();
       if (mClosed) {
         // We may have successfully read some data, but let's just throw
         // that out.