diff --git a/include/winnt.h b/include/winnt.h
index 4f65d3f22d9..b2009c40084 100644
--- a/include/winnt.h
+++ b/include/winnt.h
@@ -11,6 +11,7 @@
 #include "windef.h"
 
 #ifndef RC_INVOKED
+#include <ctype.h>
 #include <string.h>
 #endif