diff --git a/panda/src/express/checksumHashGenerator.I b/panda/src/express/checksumHashGenerator.I
index ade42717b0..640b02e415 100644
--- a/panda/src/express/checksumHashGenerator.I
+++ b/panda/src/express/checksumHashGenerator.I
@@ -13,7 +13,9 @@
 
 #ifdef _WIN32
 // Needed for PtrToLong, below
+#ifndef WIN32_LEAN_AND_MEAN
 #define WIN32_LEAN_AND_MEAN 1
+#endif
 #include <windows.h>
 #endif