diff --git a/plugins/janus_sip.c b/plugins/janus_sip.c
index dcc6da0d751..30edbe66f09 100644
--- a/plugins/janus_sip.c
+++ b/plugins/janus_sip.c
@@ -5479,11 +5479,19 @@ void janus_sip_sofia_callback(nua_event_t event, int status, char const *phrase,
 				if(status == 401) {
 					/* Get scheme/realm from 401 error */
 					sip_www_authenticate_t const* www_auth = sip->sip_www_authenticate;
+					if(www_auth == NULL) {
+						/* No WWW-Authenticate header, give up */
+						goto auth_failed;
+					}
 					scheme = www_auth->au_scheme;
 					realm = msg_params_find(www_auth->au_params, "realm=");
 				} else {
 					/* Get scheme/realm from 407 error, proxy-auth */
 					sip_proxy_authenticate_t const* proxy_auth = sip->sip_proxy_authenticate;
+					if(proxy_auth == NULL) {
+						/* No Proxy-Authenticate header, give up */
+						goto auth_failed;
+					}
 					scheme = proxy_auth->au_scheme;
 					realm = msg_params_find(proxy_auth->au_params, "realm=");
 				}
@@ -5517,6 +5525,7 @@ void janus_sip_sofia_callback(nua_event_t event, int status, char const *phrase,
 					NUTAG_AUTH(auth),
 					TAG_END());
 			} else if(status >= 400) {
+auth_failed:
 				/* Authentication failed? */
 				session->account.registration_status = janus_sip_registration_status_failed;
 				/* Cleanup registration values */