--- res_musiconhold.c	2011-04-18 12:03:08.000000000 +0200
+++ /usr/src/asterisk-trunk-r314015/res/res_musiconhold.c	2011-04-28 11:59:44.618758260 +0200
@@ -1308,13 +1308,13 @@
 			var = ast_load_realtime("musiconhold", "name", chan->musicclass, SENTINEL);
 		}
 	}
-	if (!mohclass && !var && !ast_strlen_zero(mclass)) {
+	if (!mohclass && !var && mclass && !ast_strlen_zero(mclass)) {
 		mohclass = get_mohbyname(mclass, 1, 0);
 		if (!mohclass && realtime_possible) {
 			var = ast_load_realtime("musiconhold", "name", mclass, SENTINEL);
 		}
 	}
-	if (!mohclass && !var && !ast_strlen_zero(interpclass)) {
+	if (!mohclass && !var && interpclass && !ast_strlen_zero(interpclass)) {
 		mohclass = get_mohbyname(interpclass, 1, 0);
 		if (!mohclass && realtime_possible) {
 			var = ast_load_realtime("musiconhold", "name", interpclass, SENTINEL);