diff --git a/plugins/GSdx/res/glsl_source.h b/plugins/GSdx/res/glsl_source.h
index b0635e57ae..6d1cfba025 100644
--- a/plugins/GSdx/res/glsl_source.h
+++ b/plugins/GSdx/res/glsl_source.h
@@ -1572,7 +1572,7 @@ static const char* tfx_glsl =
 	"}\n"
 	"#endif\n"
 	"\n"
-	"void fog(vec4 c, float f)\n"
+	"void fog(inout vec4 c, float f)\n"
 	"{\n"
 	"    if(PS_FOG != 0)\n"
 	"    {\n"
diff --git a/plugins/GSdx/res/tfx.glsl b/plugins/GSdx/res/tfx.glsl
index 89b72e47c8..35cc49fc45 100644
--- a/plugins/GSdx/res/tfx.glsl
+++ b/plugins/GSdx/res/tfx.glsl
@@ -1013,7 +1013,7 @@ void colclip(inout vec4 c)
 }
 #endif
 
-void fog(vec4 c, float f)
+void fog(inout vec4 c, float f)
 {
     if(PS_FOG != 0)
     {