--- main/rtp_engine.c	(Asterisk 15.4)
+++ main/rtp_engine.c	(working copy)
@@ -1539,7 +1539,7 @@
 	if (!explicit) {
 		payload = find_static_payload_type(asterisk_format, format, code);
 
-		if (payload < 0 && (!asterisk_format || ast_option_rtpusedynamic)) {
+		if (payload < 0 && (!asterisk_format || !ast_option_rtpusedynamic)) {
 			return payload;
 		}
 	}