diff --git a/winsup/cygwin/include/sys/cygwin.h b/winsup/cygwin/include/sys/cygwin.h
index d1083264b..36c3f53b1 100644
--- a/winsup/cygwin/include/sys/cygwin.h
+++ b/winsup/cygwin/include/sys/cygwin.h
@@ -49,7 +49,7 @@ struct __cygwin_perfile
 
 /* External interface stuff */
 
-typedef
+typedef enum
   {
     CW_LOCK_PINFO,
     CW_UNLOCK_PINFO,