diff --git a/panda/src/glgsg/glgsg.h b/panda/src/glgsg/glgsg.h
index 576e974791..6c8544b08e 100644
--- a/panda/src/glgsg/glgsg.h
+++ b/panda/src/glgsg/glgsg.h
@@ -67,7 +67,9 @@
 
 // Before including gl.h, need to include windows.h
 #if defined(_WIN32)
+#ifndef WIN32_LEAN_AND_MEAN
 #define WIN32_LEAN_AND_MEAN 1
+#endif
 #include <windows.h>
 #endif