diff --git a/gfx/layers/d3d9/ThebesLayerD3D9.cpp b/gfx/layers/d3d9/ThebesLayerD3D9.cpp
index cb5ec0f82841..89d8f015d896 100644
--- a/gfx/layers/d3d9/ThebesLayerD3D9.cpp
+++ b/gfx/layers/d3d9/ThebesLayerD3D9.cpp
@@ -258,6 +258,7 @@ ThebesLayerD3D9::CleanResources()
 {
   mTexture = nsnull;
   mTextureOnWhite = nsnull;
+  mValidRegion.SetEmpty();
 }
 
 void