diff --git a/gfx/layers/d3d11/CompositorD3D11.cpp b/gfx/layers/d3d11/CompositorD3D11.cpp
index 793fb245a780..aaa12dfc122e 100644
--- a/gfx/layers/d3d11/CompositorD3D11.cpp
+++ b/gfx/layers/d3d11/CompositorD3D11.cpp
@@ -466,7 +466,11 @@ CompositorD3D11::CreateRenderTargetFromSource(const gfx::IntRect &aRect,
     const IntSize& srcSize = sourceD3D11->GetSize();
     MOZ_ASSERT(srcSize.width >= 0 && srcSize.height >= 0,
                "render targets should have nonnegative sizes");
-    if (srcBox.right <= static_cast<uint32_t>(srcSize.width) &&
+    if (srcBox.left >= 0 &&
+        srcBox.top >= 0 &&
+        srcBox.left < srcBox.right &&
+        srcBox.top < srcBox.bottom &&
+        srcBox.right <= static_cast<uint32_t>(srcSize.width) &&
         srcBox.bottom <= static_cast<uint32_t>(srcSize.height)) {
       mContext->CopySubresourceRegion(texture, 0,
                                       0, 0, 0,