diff --git a/dmenu.c b/dmenu.c
index 085dc29..839f6cc 100644
--- a/dmenu.c
+++ b/dmenu.c
@@ -415,7 +415,7 @@ keypress(XKeyEvent *ev)
 	switch(ksym) {
 	default:
 insert:
-		if (!iscntrl(*buf))
+		if (!iscntrl((unsigned char)*buf))
 			insert(buf, len);
 		break;
 	case XK_Delete: