--- a/gfx/layers/d3d11/TextureD3D11.cpp
+++ b/gfx/layers/d3d11/TextureD3D11.cpp
@@ -335,20 +335,20 @@ DXGITextureHostD3D11::GetTextureSources(
   return mTextureSource.get();
 }
 
 bool
 DataTextureSourceD3D11::Update(DataSourceSurface* aSurface,
                                nsIntRegion* aDestRegion,
                                IntPoint* aSrcOffset)
 {
-  // Right now we only support null aDestRegion and aSrcOffset (which means)
-  // full surface update. Incremental update is only used on Mac so it is
-  // not clear that we ever will need to support it for D3D.
-  MOZ_ASSERT(!aDestRegion && !aSrcOffset);
+  // Right now we only support full surface update. If aDestRegion is provided,
+  // It will be ignored. Incremental update with a source offset is only used
+  // on Mac so it is not clear that we ever will need to support it for D3D.
+  MOZ_ASSERT(!aSrcOffset);
   MOZ_ASSERT(aSurface);
 
   if (!mCompositor || !mCompositor->GetDevice()) {
     return false;
   }
 
   uint32_t bpp = BytesPerPixel(aSurface->GetFormat());
   DXGI_FORMAT dxgiFormat = SurfaceFormatToDXGIFormat(aSurface->GetFormat());

--- a/gfx/layers/d3d9/TextureD3D9.cpp
+++ b/gfx/layers/d3d9/TextureD3D9.cpp
@@ -371,20 +371,20 @@ DataTextureSourceD3D9::GetD3D9Texture()
                     : mTexture;
 }
 
 bool
 DataTextureSourceD3D9::Update(gfx::DataSourceSurface* aSurface,
                               nsIntRegion* aDestRegion,
                               gfx::IntPoint* aSrcOffset)
 {
-  // Right now we only support null aDestRegion and aSrcOffset (which means
-  // full surface update). Incremental update is only used on Mac so it is
-  // not clear that we ever will need to support it for D3D.
-  MOZ_ASSERT(!aDestRegion && !aSrcOffset);
+  // Right now we only support full surface update. If aDestRegion is provided,
+  // It will be ignored. Incremental update with a source offset is only used
+  // on Mac so it is not clear that we ever will need to support it for D3D.
+  MOZ_ASSERT(!aSrcOffset);
 
   if (!mCompositor || !mCompositor->device()) {
     NS_WARNING("No D3D device to update the texture.");
     return false;
   }
   mSize = aSurface->GetSize();
 
   uint32_t bpp = 0;