diff --git a/src/sip/transp.c b/src/sip/transp.c
index 8c217a789..ea9e9d43c 100644
--- a/src/sip/transp.c
+++ b/src/sip/transp.c
@@ -314,7 +314,8 @@ static bool have_essential_fields(const struct sip_msg *msg)
 		pl_isset(&(msg->from.auri)) &&
 		pl_isset(&(msg->cseq.met)) &&
 		pl_isset(&(msg->callid)) &&
-		(pl_isset(&(msg->maxfwd)) || !pl_strcmp(&msg->met, "ACK")) &&
+		(pl_isset(&(msg->maxfwd)) ||
+		 !pl_strncmp(&msg->met, "ACK", 3)) &&
 		pl_isset(&(msg->via.branch)))
 		return true;