--- chan_sip.c~	2018-01-11 17:44:54.000000000 +0100
+++ chan_sip.c	2018-02-12 16:27:58.572141588 +0100
@@ -27174,7 +27174,7 @@
 	} else {
 		sip_scheddestroy(p, DEFAULT_TRANS_TIMEOUT);
 	}
-	if (ast_str_strlen(p->initreq.data) > 0) {
+	if (p->initreq.data && ast_str_strlen(p->initreq.data) > 0) {
 		struct sip_pkt *pkt, *prev_pkt;
 		/* If the CANCEL we are receiving is a retransmission, and we already have scheduled
 		 * a reliable 487, then we don't want to schedule another one on top of the previous