diff --git a/slock.c b/slock.c
index f16781f..3cede0f 100644
--- a/slock.c
+++ b/slock.c
@@ -181,7 +181,7 @@ readpw(Display *dpy, struct xrandr *rr, struct lock **locks, int nscreens,
 					passwd[--len] = '\0';
 				break;
 			default:
-				if (num && !iscntrl((int)buf[0]) &&
+				if (num && !iscntrl((unsigned char)buf[0]) &&
 				    (len + num < sizeof(passwd))) {
 					memcpy(passwd + len, buf, num);
 					len += num;