diff --git a/src/wormhole_mailbox_server/server.py b/src/wormhole_mailbox_server/server.py
index 6f8bf99..3c96a82 100644
--- a/src/wormhole_mailbox_server/server.py
+++ b/src/wormhole_mailbox_server/server.py
@@ -11,6 +11,8 @@ class CrowdedError(Exception):
     pass
 class ReclaimedError(Exception):
     pass
+class UnknownNameplateError(Exception):
+    pass
 
 Usage = namedtuple("Usage", ["started", "waiting_time", "total_time", "result"])
 TransitUsage = namedtuple("TransitUsage",
@@ -236,7 +238,7 @@ def allocate_nameplate(self, side, when):
         mailbox_id = self.claim_nameplate(nameplate_id, side, when)
         return nameplate_id, mailbox_id
 
-    def claim_nameplate(self, name, side, when):
+    def claim_nameplate(self, name, side, when, allow_allocate=True):
         # when we're done:
         # * there will be one row for the nameplate
         #  * there will be one 'side' attached to it, with claimed=True
@@ -249,6 +251,8 @@ def claim_nameplate(self, name, side, when):
                          " WHERE `app_id`=? AND `name`=?",
                          (self._app_id, name)).fetchone()
         if not row:
+            if not allow_allocate:
+                raise UnknownNameplateError("Claimed nameplate does not exist, and `allow_allocate` is set to False")
             if self._log_requests:
                 log.msg("creating nameplate#%s for app_id %s" %
                         (name, self._app_id))
diff --git a/src/wormhole_mailbox_server/server_websocket.py b/src/wormhole_mailbox_server/server_websocket.py
index 45ed440..b2bfa67 100644
--- a/src/wormhole_mailbox_server/server_websocket.py
+++ b/src/wormhole_mailbox_server/server_websocket.py
@@ -3,7 +3,7 @@
 from twisted.internet import reactor
 from twisted.python import log
 from autobahn.twisted import websocket
-from .server import CrowdedError, ReclaimedError, SidedMessage
+from .server import CrowdedError, ReclaimedError, UnknownNameplateError, SidedMessage
 from .util import dict_to_bytes, bytes_to_dict
 
 # The WebSocket allows the client to send "commands" to the server, and the
@@ -193,6 +193,10 @@ def handle_allocate(self, server_rx):
     def handle_claim(self, msg, server_rx):
         if "nameplate" not in msg:
             raise Error("claim requires 'nameplate'")
+        allow_allocate = True
+        if "allocate" in msg:
+            allow_allocate = msg["allocate"]
+
         if self._did_claim:
             raise Error("only one claim per connection")
         self._did_claim = True
@@ -201,11 +205,13 @@ def handle_claim(self, msg, server_rx):
         self._nameplate_id = nameplate_id
         try:
             mailbox_id = self._app.claim_nameplate(nameplate_id, self._side,
-                                                   server_rx)
+                                                   server_rx, allow_allocate)
         except CrowdedError:
             raise Error("crowded")
         except ReclaimedError:
             raise Error("reclaimed")
+        except UnknownNameplateError:
+            raise Error("unallocated")
         self.send("claimed", mailbox=mailbox_id)
 
     def handle_release(self, msg, server_rx):
diff --git a/src/wormhole_mailbox_server/test/test_web.py b/src/wormhole_mailbox_server/test/test_web.py
index ecf462b..f2c45c9 100644
--- a/src/wormhole_mailbox_server/test/test_web.py
+++ b/src/wormhole_mailbox_server/test/test_web.py
@@ -280,6 +280,11 @@ def test_claim(self):
         self.assertEqual(err["type"], "error")
         self.assertEqual(err["error"], "claim requires 'nameplate'")
 
+        c1.send("claim", nameplate="np1", allocate=False) # not allocated
+        err = yield c1.next_non_ack()
+        self.assertEqual(err["type"], "error")
+        self.assertEqual(err["error"], "unallocated")
+
         c1.send("claim", nameplate="np1")
         m = yield c1.next_non_ack()
         self.assertEqual(m["type"], "claimed")