--- a/dom/media/MediaData.h
+++ b/dom/media/MediaData.h
@@ -218,18 +218,18 @@ private:
   // aligned and that it has sufficient end padding to allow for Alignment bytes
   // block read as required by some data decoders.
   // Returns false if memory couldn't be allocated.
   bool EnsureCapacity(size_t aLength)
   {
     const CheckedInt<size_t> sizeNeeded =
       CheckedInt<size_t>(aLength) * sizeof(Type) + AlignmentPaddingSize();
 
-    if (!sizeNeeded.isValid()) {
-      // overflow.
+    if (!sizeNeeded.isValid() || sizeNeeded.value() >= INT32_MAX) {
+      // overflow or over an acceptable size.
       return false;
     }
     if (mData && mCapacity >= sizeNeeded.value()) {
       return true;
     }
     auto newBuffer = MakeUniqueFallible<uint8_t[]>(sizeNeeded.value());
     if (!newBuffer) {
       return false;