diff -urN asterisk-1.4.22-orig/channels/chan_sip.c asterisk-1.4.22/channels/chan_sip.c
--- asterisk-1.4.22-orig/channels/chan_sip.c	2008-09-08 23:10:10.000000000 +0200
+++ asterisk-1.4.22/channels/chan_sip.c	2008-10-31 00:56:15.000000000 +0100
@@ -14246,7 +14246,9 @@
 			ast_mutex_unlock(&p->lock);
 			if (p->refer->refer_call) {
 				ast_mutex_unlock(&p->refer->refer_call->lock);
-				ast_channel_unlock(p->refer->refer_call->owner);
+				if(p->refer->refer_call->owner) {
+					ast_channel_unlock(p->refer->refer_call->owner);
+				}
 			}
 			p->invitestate = INV_COMPLETED;
 			return -1;