diff --git a/reactos/w32api/include/winnt.h b/reactos/w32api/include/winnt.h
index 8c0cc56bf79..6409dd3ecf9 100644
--- a/reactos/w32api/include/winnt.h
+++ b/reactos/w32api/include/winnt.h
@@ -34,6 +34,7 @@
 extern "C" {
 #endif
 
+#include <ctype.h>
 #include <winerror.h>
 
 #ifndef RC_INVOKED