diff --git a/dom/media/MediaResource.h b/dom/media/MediaResource.h
index b32999f48cce..9badc24ab3a5 100644
--- a/dom/media/MediaResource.h
+++ b/dom/media/MediaResource.h
@@ -747,7 +747,9 @@ 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))