diff --git a/appl/xnlock/xnlock.c b/appl/xnlock/xnlock.c
index 31ae60e36..e2ce4dbcc 100644
--- a/appl/xnlock/xnlock.c
+++ b/appl/xnlock/xnlock.c
@@ -759,7 +759,7 @@ GetPasswd(Widget w, XEvent *_event, String *_s, Cardinal *_n)
 			 prompt_x + XTextWidth(font, STRING, echolen),
 			 prompt_y, SPACE_STRING, STRING_LENGTH - echolen + 1);
       }
-    } else if (isprint(c)) {
+    } else if (isprint((unsigned char)c)) {
 	if ((cnt + 1) >= MAX_PASSWD_LENGTH)
 	    XBell(dpy, 50);
 	else