diff --git a/src/wormhole/blocking/transcribe.py b/src/wormhole/blocking/transcribe.py
index f19c18c..09292f6 100644
--- a/src/wormhole/blocking/transcribe.py
+++ b/src/wormhole/blocking/transcribe.py
@@ -135,7 +135,7 @@ class Wormhole:
         return channel_id
 
     def derive_key(self, purpose, length=SecretBox.KEY_SIZE):
-        if type(purpose) is not type(b""): raise UsageError
+        if not isinstance(purpose, type(b"")): raise UsageError
         return HKDF(self.key, length, CTXinfo=purpose)
 
 
diff --git a/src/wormhole/twisted/transcribe.py b/src/wormhole/twisted/transcribe.py
index c8e7f9b..9d4be6a 100644
--- a/src/wormhole/twisted/transcribe.py
+++ b/src/wormhole/twisted/transcribe.py
@@ -200,7 +200,7 @@ class SymmetricWormhole:
         if self.key is None:
             # call after get_verifier() or get_data()
             raise UsageError
-        if type(purpose) is not type(b""): raise UsageError
+        if not isinstance(purpose, type(b"")): raise UsageError
         return HKDF(self.key, length, CTXinfo=purpose)
 
     def _encrypt_data(self, key, data):