--- a/dom/media/MediaResource.h
+++ b/dom/media/MediaResource.h
@@ -764,17 +764,19 @@ private:
  */
 
 class MediaResourceIndex
 {
 public:
   explicit MediaResourceIndex(MediaResource* aResource)
     : mResource(aResource)
     , mOffset(0)
-    , mCacheBlockSize(SelectCacheSize(MediaPrefs::MediaResourceIndexCache()))
+    , mCacheBlockSize(aResource->ShouldCacheReads()
+                      ? SelectCacheSize(MediaPrefs::MediaResourceIndexCache())
+                      : 0 )
     , mCachedOffset(0)
     , mCachedBytes(0)
     , mCachedBlock(MakeUnique<char[]>(mCacheBlockSize))
   {}
 
   // Read up to aCount bytes from the stream. The buffer must have
   // enough room for at least aCount bytes. Stores the number of
   // actual bytes read in aBytes (0 on end of file).