--- a/dom/media/BufferMediaResource.h
+++ b/dom/media/BufferMediaResource.h
@@ -70,17 +70,20 @@ private:
   }
   int64_t Tell() override { return mOffset; }
 
   void Pin() override {}
   void Unpin() override {}
   double GetDownloadRate(bool* aIsReliable) override { *aIsReliable = false; return 0.; }
   int64_t GetLength() override { return mLength; }
   int64_t GetNextCachedData(int64_t aOffset) override { return aOffset; }
-  int64_t GetCachedDataEnd(int64_t aOffset) override { return mLength; }
+  int64_t GetCachedDataEnd(int64_t aOffset) override
+  {
+    return std::max(aOffset, int64_t(mLength));
+  }
   bool IsDataCachedToEndOfResource(int64_t aOffset) override { return true; }
   bool IsSuspendedByCache() override { return false; }
   bool IsSuspended() override { return false; }
   nsresult ReadFromCache(char* aBuffer,
                          int64_t aOffset,
                          uint32_t aCount) override
   {
     if (aOffset < 0) {