diff --git a/net.c b/net.c
index dd3962e..e47f727 100644
--- a/net.c
+++ b/net.c
@@ -246,6 +246,7 @@
         QTAILQ_INSERT_TAIL(&vc->vlan->clients, vc, next);
     } else {
         if (peer) {
+            assert(!peer->peer);
             vc->peer = peer;
             peer->peer = vc;
         }