--- a/gfx/layers/client/TiledContentClient.cpp
+++ b/gfx/layers/client/TiledContentClient.cpp
@@ -1277,18 +1277,18 @@ ClientTiledLayerBuffer::ValidateTile(Til
 
   ctxt = nullptr;
   drawTarget = nullptr;
 
   nsIntRegion tileRegion =
     nsIntRect(aTileOrigin.x, aTileOrigin.y,
               GetScaledTileSize().width, GetScaledTileSize().height);
   // Intersect this area with the portion that's invalid.
-  tileRegion = tileRegion.Sub(tileRegion, GetValidRegion());
-  tileRegion = tileRegion.Sub(tileRegion, aDirtyRegion); // Has now been validated
+  tileRegion.SubOut(GetValidRegion());
+  tileRegion.SubOut(aDirtyRegion); // Has now been validated
 
   backBuffer->SetWaste(tileRegion.Area() * mResolution * mResolution);
   backBuffer->Unlock();
 
   if (createdTextureClient) {
     if (!mCompositableClient->AddTextureClient(backBuffer)) {
       NS_WARNING("Failed to add tile TextureClient.");
       aTile.DiscardFrontBuffer();