Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 145191)
+++ channels/chan_sip.c	(working copy)
@@ -20269,6 +20269,8 @@
 				ast_log(LOG_WARNING, "'%s' is not a valid transport option to Dial() for SIP calls, using udp by default.\n", trans);
 			transport = SIP_TRANSPORT_UDP;
 		}
+	} else { /* use default */
+		transport = SIP_TRANSPORT_UDP;
 	}
 
 	if (!host) {