diff --git a/src/msw/ole/automtn.cpp b/src/msw/ole/automtn.cpp
index cc3dda234b..81c563dc3b 100644
--- a/src/msw/ole/automtn.cpp
+++ b/src/msw/ole/automtn.cpp
@@ -26,7 +26,10 @@
     #include "wx/math.h"
 #endif
 
-#define _FORCENAMELESSUNION
+#ifndef _FORCENAMELESSUNION
+    #define _FORCENAMELESSUNION
+#endif
+
 #include "wx/msw/private.h"
 #include "wx/msw/ole/oleutils.h"
 #include "wx/msw/ole/automtn.h"