diff --git a/src/core/tcp.c b/src/core/tcp.c
index 3cbde688..31333a51 100644
--- a/src/core/tcp.c
+++ b/src/core/tcp.c
@@ -593,17 +593,14 @@ tcp_listen_dual_with_backlog(struct tcp_pcb *pcb, u8_t backlog)
   struct tcp_pcb *lpcb;
   struct tcp_pcb_listen *l;
 
-  /* Dual listen only works on bound pcbs (or else we cannot check for
-      duplicate port use. */
-  if (pcb->local_port == 0) {
-    return NULL;
-  }
-  /* Check that there's noone listening on this port already
-     (don't check the IP address since we'll set it to ANY */
-  for(l = tcp_listen_pcbs.listen_pcbs; l != NULL; l = l->next) {
-    if (l->local_port == pcb->local_port) {
-      /* this port is already used */
-      return NULL;
+  if (pcb->local_port != 0) {
+    /* Check that there's noone listening on this port already
+       (don't check the IP address since we'll set it to ANY */
+    for(l = tcp_listen_pcbs.listen_pcbs; l != NULL; l = l->next) {
+      if (l->local_port == pcb->local_port) {
+        /* this port is already used */
+        return NULL;
+      }
     }
   }