diff -up dwm-5.1/dwm.c dwm-5.1-patched_by_zuo/dwm.c
--- dwm-5.1/dwm.c	2008-07-29 20:18:32.000000000 +0200
+++ dwm-5.1-patched_by_zuo/dwm.c	2008-08-22 02:56:19.000000000 +0200
@@ -1497,9 +1497,12 @@ togglefloating(const Arg *arg) {
 
 void
 toggletag(const Arg *arg) {
+	if(!sel)
+		return;
+
 	unsigned int mask = sel->tags ^ (arg->ui & TAGMASK);
 
-	if(sel && mask) {
+	if(mask) {
 		sel->tags = mask;
 		arrange();
 	}