--- channels/chan_sip.c	(version 13.4.0)
+++ channels/chan_sip.c	(working copy)
@@ -30408,4 +30408,7 @@
 		}
-		if (!(peer->the_mark))
+		if (!(peer->the_mark)) {
 			firstpass = 0;
+		} else {
+			ast_format_cap_remove_by_type(peer->caps, AST_MEDIA_TYPE_UNKNOWN);
+		}
 	} else {