diff --git a/port/fast3d/gfx_pc.cpp b/port/fast3d/gfx_pc.cpp
index 17915be6b..b74313fc4 100644
--- a/port/fast3d/gfx_pc.cpp
+++ b/port/fast3d/gfx_pc.cpp
@@ -72,7 +72,7 @@ struct RGBA {
 struct NormalColor {
     union {
         struct { uint8_t r, g, b, a; };
-        struct { uint8_t x, y, z, w; };
+        struct { int8_t x, y, z, w; };
     };
 };