diff --git a/src/interfaces.h b/src/interfaces.h
index 98af39f4..e585c8a3 100644
--- a/src/interfaces.h
+++ b/src/interfaces.h
@@ -46,6 +46,7 @@ class IVModelRender;
 class IVModelRender;
 class IVRenderView;
 class IMaterialSystemFixed;
+class IMaterialSystem;
 
 extern ISteamClient* g_ISteamClient;
 extern ISteamFriends* g_ISteamFriends;