diff --git a/libavformat/udp.c b/libavformat/udp.c
index dfc3b5aaab..04e60e3793 100644
--- a/libavformat/udp.c
+++ b/libavformat/udp.c
@@ -519,7 +519,7 @@ static int udp_open(URLContext *h, const char *uri, int flags)
             goto fail;
     }
 
-    if ((s->is_multicast || !s->local_port) && (h->flags & AVIO_FLAG_READ))
+    if ((s->is_multicast || s->local_port < 0) && (h->flags & AVIO_FLAG_READ))
         s->local_port = port;
 
     if (localaddr[0])