diff --git a/libagent/device/trezor.py b/libagent/device/trezor.py
index 5d7cccf..0dc3d5b 100644
--- a/libagent/device/trezor.py
+++ b/libagent/device/trezor.py
@@ -79,7 +79,7 @@ class Trezor(interface.Device):
 
         conn.callback_PinMatrixRequest = new_handler
 
-    cached_passphrase_ack = 0
+    cached_passphrase_ack = None
 
     def _override_passphrase_handler(self, conn):
         cli_handler = conn.callback_PassphraseRequest