diff --git a/src/wormhole/blocking/transcribe.py b/src/wormhole/blocking/transcribe.py
index bdd4643..fdca19b 100644
--- a/src/wormhole/blocking/transcribe.py
+++ b/src/wormhole/blocking/transcribe.py
@@ -185,6 +185,8 @@ class Wormhole:
             self.verifier = self.derive_key(self.appid+b":Verifier")
 
     def get_verifier(self):
+        if self.code is None: raise UsageError
+        if self.channel_id is None: raise UsageError
         self._get_key()
         return self.verifier
 
diff --git a/src/wormhole/test/test_twisted.py b/src/wormhole/test/test_twisted.py
index 815facc..8e267fc 100644
--- a/src/wormhole/test/test_twisted.py
+++ b/src/wormhole/test/test_twisted.py
@@ -1,16 +1,18 @@
 import json
 from twisted.trial import unittest
 from twisted.internet import defer
+from twisted.internet.threads import deferToThread
 from twisted.application import service
 from ..servers.relay import RelayServer
 from ..twisted.transcribe import Wormhole, UsageError
 from ..twisted.util import allocate_ports
+from ..blocking.transcribe import Wormhole as BlockingWormhole
 from .. import __version__
 #from twisted.python import log
 #import sys
 #log.startLogging(sys.stdout)
 
-class Basic(unittest.TestCase):
+class ServerBase:
     def setUp(self):
         self.sp = service.MultiService()
         self.sp.startService()
@@ -29,6 +31,7 @@ class Basic(unittest.TestCase):
     def tearDown(self):
         return self.sp.stopService()
 
+class Basic(ServerBase, unittest.TestCase):
     def test_basic(self):
         appid = "appid"
         w1 = Wormhole(appid, self.relayurl)
@@ -111,3 +114,93 @@ class Basic(unittest.TestCase):
             self.assertRaises(UsageError, w2.serialize) # too late
         d.addCallback(_done)
         return d
+
+class Blocking(ServerBase, unittest.TestCase):
+    # we need Twisted to run the server, but we run the sender and receiver
+    # with deferToThread()
+
+    def test_basic(self):
+        appid = "appid"
+        w1 = BlockingWormhole(appid, self.relayurl)
+        w2 = BlockingWormhole(appid, self.relayurl)
+        d = deferToThread(w1.get_code)
+        def _got_code(code):
+            w2.set_code(code)
+            d1 = deferToThread(w1.get_data, "data1")
+            d2 = deferToThread(w2.get_data, "data2")
+            return defer.DeferredList([d1,d2], fireOnOneErrback=False)
+        d.addCallback(_got_code)
+        def _done(dl):
+            ((success1, dataX), (success2, dataY)) = dl
+            r1,r2 = dl
+            self.assertTrue(success1, dataX)
+            self.assertTrue(success2, dataY)
+            self.assertEqual(dataX, "data2")
+            self.assertEqual(dataY, "data1")
+        d.addCallback(_done)
+        return d
+
+    def test_fixed_code(self):
+        appid = "appid"
+        w1 = BlockingWormhole(appid, self.relayurl)
+        w2 = BlockingWormhole(appid, self.relayurl)
+        w1.set_code("123-purple-elephant")
+        w2.set_code("123-purple-elephant")
+        d1 = deferToThread(w1.get_data, "data1")
+        d2 = deferToThread(w2.get_data, "data2")
+        d = defer.DeferredList([d1,d2], fireOnOneErrback=False)
+        def _done(dl):
+            ((success1, dataX), (success2, dataY)) = dl
+            r1,r2 = dl
+            self.assertTrue(success1, dataX)
+            self.assertTrue(success2, dataY)
+            self.assertEqual(dataX, "data2")
+            self.assertEqual(dataY, "data1")
+        d.addCallback(_done)
+        return d
+
+    def test_errors(self):
+        appid = "appid"
+        w1 = BlockingWormhole(appid, self.relayurl)
+        self.assertRaises(UsageError, w1.get_verifier)
+        self.assertRaises(UsageError, w1.get_data, "data")
+        w1.set_code("123-purple-elephant")
+        self.assertRaises(UsageError, w1.set_code, "123-nope")
+        self.assertRaises(UsageError, w1.get_code)
+        w2 = BlockingWormhole(appid, self.relayurl)
+        d = deferToThread(w2.get_code)
+        def _done(code):
+            self.assertRaises(UsageError, w2.get_code)
+        d.addCallback(_done)
+        return d
+
+    def test_serialize(self):
+        appid = "appid"
+        w1 = BlockingWormhole(appid, self.relayurl)
+        self.assertRaises(UsageError, w1.serialize) # too early
+        w2 = BlockingWormhole(appid, self.relayurl)
+        d = deferToThread(w1.get_code)
+        def _got_code(code):
+            self.assertRaises(UsageError, w2.serialize) # too early
+            w2.set_code(code)
+            w2.serialize() # ok
+            s = w1.serialize()
+            self.assertEqual(type(s), type(""))
+            unpacked = json.loads(s) # this is supposed to be JSON
+            self.assertEqual(type(unpacked), dict)
+            new_w1 = BlockingWormhole.from_serialized(s)
+            d1 = deferToThread(new_w1.get_data, "data1")
+            d2 = deferToThread(w2.get_data, "data2")
+            return defer.DeferredList([d1,d2], fireOnOneErrback=False)
+        d.addCallback(_got_code)
+        def _done(dl):
+            ((success1, dataX), (success2, dataY)) = dl
+            r1,r2 = dl
+            self.assertTrue(success1, dataX)
+            self.assertTrue(success2, dataY)
+            self.assertEqual(dataX, "data2")
+            self.assertEqual(dataY, "data1")
+            self.assertRaises(UsageError, w2.serialize) # too late
+        d.addCallback(_done)
+        return d
+    test_serialize.skip = "not yet implemented for the blocking flavor"