diff --git a/gfx/layers/d3d9/CompositorD3D9.cpp b/gfx/layers/d3d9/CompositorD3D9.cpp
index 8774131ec07..3cd28b55ff1 100644
--- a/gfx/layers/d3d9/CompositorD3D9.cpp
+++ b/gfx/layers/d3d9/CompositorD3D9.cpp
@@ -692,6 +692,7 @@ CompositorD3D9::PrepareViewport(const gfx::IntSize& aSize)
   viewMatrix._22 = -2.0f / aSize.height;
   viewMatrix._41 = -1.0f;
   viewMatrix._42 = 1.0f;
+  viewMatrix._33 = 0.0f;
 
   HRESULT hr = device()->SetVertexShaderConstantF(CBmProjection, &viewMatrix._11, 4);