diff --git a/src/wormhole/server/rendezvous_websocket.py b/src/wormhole/server/rendezvous_websocket.py
index dc4eb51..a5f8eb0 100644
--- a/src/wormhole/server/rendezvous_websocket.py
+++ b/src/wormhole/server/rendezvous_websocket.py
@@ -231,8 +231,11 @@ class WebSocketRendezvous(websocket.WebSocketServerProtocol):
         mailbox_id = msg["mailbox"]
         assert isinstance(mailbox_id, type(""))
         self._mailbox_id = mailbox_id
-        self._mailbox = self._app.open_mailbox(mailbox_id, self._side,
-                                               server_rx)
+        try:
+            self._mailbox = self._app.open_mailbox(mailbox_id, self._side,
+                                                   server_rx)
+        except CrowdedError:
+            raise Error("crowded")
         def _send(sm):
             self.send("message", side=sm.side, phase=sm.phase,
                       body=sm.body, server_rx=sm.server_rx, id=sm.msg_id)
@@ -268,8 +271,11 @@ class WebSocketRendezvous(websocket.WebSocketServerProtocol):
                 raise Error("close without mailbox must follow open")
             mailbox_id = self._mailbox_id
         if not self._mailbox:
-            self._mailbox = self._app.open_mailbox(mailbox_id, self._side,
-                                                   server_rx)
+            try:
+                self._mailbox = self._app.open_mailbox(mailbox_id, self._side,
+                                                       server_rx)
+            except CrowdedError:
+                raise Error("crowded")
         if self._listening:
             self._mailbox.remove_listener(self)
             self._listening = False
diff --git a/src/wormhole/test/test_server.py b/src/wormhole/test/test_server.py
index f27eb9f..514d573 100644
--- a/src/wormhole/test/test_server.py
+++ b/src/wormhole/test/test_server.py
@@ -892,6 +892,22 @@ class WebSocketAPI(_Util, ServerBase, unittest.TestCase):
         self.assertEqual(err["type"], "error")
         self.assertEqual(err["error"], "only one open per connection")
 
+    @inlineCallbacks
+    def test_open_crowded(self):
+        c1 = yield self.make_client()
+        yield c1.next_non_ack()
+        c1.send("bind", appid="appid", side="side")
+        app = self._rendezvous.get_app("appid")
+
+        mbid = app.claim_nameplate("np1", "side1", 0)
+        app.claim_nameplate("np1", "side2", 0)
+
+        # the third open will signal crowding
+        c1.send("open", mailbox=mbid)
+        err = yield c1.next_non_ack()
+        self.assertEqual(err["type"], "error")
+        self.assertEqual(err["error"], "crowded")
+
     @inlineCallbacks
     def test_add(self):
         c1 = yield self.make_client()
@@ -991,6 +1007,22 @@ class WebSocketAPI(_Util, ServerBase, unittest.TestCase):
         self.assertEqual(err["type"], "error")
         self.assertEqual(err["error"], "open and close must use same mailbox")
 
+    @inlineCallbacks
+    def test_close_crowded(self):
+        c1 = yield self.make_client()
+        yield c1.next_non_ack()
+        c1.send("bind", appid="appid", side="side")
+        app = self._rendezvous.get_app("appid")
+
+        mbid = app.claim_nameplate("np1", "side1", 0)
+        app.claim_nameplate("np1", "side2", 0)
+
+        # a close that allocates a third side will signal crowding
+        c1.send("close", mailbox=mbid)
+        err = yield c1.next_non_ack()
+        self.assertEqual(err["type"], "error")
+        self.assertEqual(err["error"], "crowded")
+
 
     @inlineCallbacks
     def test_disconnect(self):