diff --git a/src/wormhole/twisted/transit.py b/src/wormhole/twisted/transit.py
index fc3fea5..e5c684f 100644
--- a/src/wormhole/twisted/transit.py
+++ b/src/wormhole/twisted/transit.py
@@ -478,18 +478,21 @@ class Common:
         self._their_relay_hints = set(hints)
 
     def _send_this(self):
+        assert self._transit_key
         if self.is_sender:
             return build_sender_handshake(self._transit_key)
         else:
             return build_receiver_handshake(self._transit_key)
 
     def _expect_this(self):
+        assert self._transit_key
         if self.is_sender:
             return build_receiver_handshake(self._transit_key)
         else:
             return build_sender_handshake(self._transit_key)# + b"go\n"
 
     def _sender_record_key(self):
+        assert self._transit_key
         if self.is_sender:
             return HKDF(self._transit_key, SecretBox.KEY_SIZE,
                         CTXinfo=b"transit_record_sender_key")
@@ -498,6 +501,7 @@ class Common:
                         CTXinfo=b"transit_record_receiver_key")
 
     def _receiver_record_key(self):
+        assert self._transit_key
         if self.is_sender:
             return HKDF(self._transit_key, SecretBox.KEY_SIZE,
                         CTXinfo=b"transit_record_receiver_key")
@@ -506,6 +510,7 @@ class Common:
                         CTXinfo=b"transit_record_sender_key")
 
     def set_transit_key(self, key):
+        assert isinstance(key, type(b"")), type(key)
         # We use pubsub to protect against the race where the sender knows
         # the hints and the key, and connects to the receiver's transit
         # socket before the receiver gets the relay message (and thus the
@@ -589,6 +594,7 @@ class Common:
     def _start_connector(self, ep, description, is_relay=False):
         relay_handshake = None
         if is_relay:
+            assert self._transit_key
             relay_handshake = build_relay_handshake(self._transit_key)
         f = OutboundConnectionFactory(self, relay_handshake)
         d = ep.connect(f)
diff --git a/src/wormhole/util/hkdf.py b/src/wormhole/util/hkdf.py
index 3f6f9ad..c3a477d 100644
--- a/src/wormhole/util/hkdf.py
+++ b/src/wormhole/util/hkdf.py
@@ -4,9 +4,9 @@ import six
 
 def HKDF(SKM, dkLen, XTS=None, CTXinfo=b"", digest=sha256,
          _test_expected_PRK=None):
-    assert isinstance(SKM, six.binary_type)
-    assert isinstance(XTS, (six.binary_type,type(None)))
-    assert isinstance(CTXinfo, six.binary_type)
+    assert isinstance(SKM, six.binary_type), type(SKM)
+    assert isinstance(XTS, (six.binary_type,type(None))), type(XTS)
+    assert isinstance(CTXinfo, six.binary_type), type(CTXinfo)
     hlen = len(digest(b"").digest())
     assert dkLen <= hlen*255
     if XTS is None: