diff --git a/reactos/lib/oleaut32/vartype.c b/reactos/lib/oleaut32/vartype.c
index 2e771e8ce75..ebdaf5c09ed 100644
--- a/reactos/lib/oleaut32/vartype.c
+++ b/reactos/lib/oleaut32/vartype.c
@@ -29,6 +29,7 @@
 #include "winnt.h"
 #include "variant.h"
 #include "resource.h"
+#include <ctype.h>
 
 WINE_DEFAULT_DEBUG_CHANNEL(variant);