diff --git a/configure b/configure
index e285e16545..81f056f695 100755
--- a/configure
+++ b/configure
@@ -3035,6 +3035,7 @@ check_cflags -Wdeclaration-after-statement
 check_cflags -Wall
 check_cflags -Wno-parentheses
 check_cflags -Wno-switch
+check_cflags -Wno-format-zero-length
 check_cflags -Wdisabled-optimization
 check_cflags -Wpointer-arith
 check_cflags -Wredundant-decls
diff --git a/libavformat/udp.c b/libavformat/udp.c
index 7c18fb7bf0..21b4455948 100644
--- a/libavformat/udp.c
+++ b/libavformat/udp.c
@@ -444,7 +444,7 @@ static int udp_open(URLContext *h, const char *uri, int flags)
             goto fail;
     }
 
-    if (s->is_multicast && (h->flags & AVIO_FLAG_READ))
+    if ((s->is_multicast || !s->local_port) && (h->flags & AVIO_FLAG_READ))
         s->local_port = port;
     udp_fd = udp_socket_create(s, &my_addr, &len);
     if (udp_fd < 0)