diff --git a/src/Renderer/QuadRasterizer.cpp b/src/Renderer/QuadRasterizer.cpp
index 691d325..f88c39e 100644
--- a/src/Renderer/QuadRasterizer.cpp
+++ b/src/Renderer/QuadRasterizer.cpp
@@ -332,11 +332,11 @@
 
 	bool QuadRasterizer::interpolateZ() const
 	{
-		return state.depthTestActive || state.pixelFogActive() || (shader && shader->vPosDeclared && fullPixelPositionRegister);
+		return state.depthTestActive || state.pixelFogActive() || (shader && shader->isVPosDeclared() && fullPixelPositionRegister);
 	}
 
 	bool QuadRasterizer::interpolateW() const
 	{
-		return state.perspective || (shader && shader->vPosDeclared && fullPixelPositionRegister);
+		return state.perspective || (shader && shader->isVPosDeclared() && fullPixelPositionRegister);
 	}
 }