diff --git a/common/mathlib.h b/common/mathlib.h
index d068176b..65b750f5 100644
--- a/common/mathlib.h
+++ b/common/mathlib.h
@@ -42,6 +42,8 @@ typedef vec_t vec4_t[4];	// x,y,z,w
 #define M_PI		3.14159265358979323846	// matches value in gcc v2 math.h
 #endif
 
+struct mplane_s;
+
 extern vec3_t vec3_origin;
 extern	int nanmask;