diff --git a/pandatool/src/maxegg/maxNodeTree.cxx b/pandatool/src/maxegg/maxNodeTree.cxx
index 3da9b97fbc..eb2e973927 100755
--- a/pandatool/src/maxegg/maxNodeTree.cxx
+++ b/pandatool/src/maxegg/maxNodeTree.cxx
@@ -15,6 +15,11 @@
 
 #include "maxEgg.h"
 
+// 3dsmax versions below 9 don't define this.
+#ifndef _M
+#define _M(s) (s)
+#endif
+
 ////////////////////////////////////////////////////////////////////
 //     Function: MaxNodeTree::Constructor
 //       Access: Public
@@ -486,4 +491,4 @@ void MaxNodeTree::set_collision_tags(MaxNodeDesc *node_desc, EggGroup *egg_group
       }
     }
     return;
-}
\ No newline at end of file
+}