diff --git a/dwm.c b/dwm.c
index f26a943..212c6ba 100644
--- a/dwm.c
+++ b/dwm.c
@@ -1955,7 +1955,8 @@ updatenumlockmask(void)
 	XModifierKeymap *modmap;
 
 	numlockmask = 0;
-	modmap = XGetModifierMapping(dpy);
+	if (!(modmap = XGetModifierMapping(dpy)))
+		return;
 	for (i = 0; i < 8; i++)
 		for (j = 0; j < modmap->max_keypermod; j++)
 			if (modmap->modifiermap[i * modmap->max_keypermod + j]