--- a/dwm.c	Tue Apr 08 11:49:35 2008 +0100
+++ b/dwm.c	Sun Apr 20 18:17:33 2008 +0100
@@ -262,8 +262,8 @@
 	for(i = 0; i < LENGTH(rules); i++) {
 		r = &rules[i];
 		if((!r->title || strstr(c->name, r->title))
-		&& (!ch.res_class || !r->class || strstr(ch.res_class, r->class))
-		&& (!ch.res_name || !r->instance || strstr(ch.res_name, r->instance)))
+		&& (!r->class || (ch.res_class && strstr(ch.res_class, r->class)))
+		&& (!r->instance || (ch.res_name && strstr(ch.res_name, r->instance)))) {
 		{
 			c->isfloating = r->isfloating;
 			if(r->tag) {