diff --git a/sshagent/formats.py b/sshagent/formats.py
index f8ce6eb..f6bc62f 100644
--- a/sshagent/formats.py
+++ b/sshagent/formats.py
@@ -64,6 +64,7 @@ def decompress_pubkey(pub, curve=ecdsa.NIST256p):
     return ecdsa.VerifyingKey.from_public_point(point, curve=curve,
                                                 hashfunc=hashfunc)
 
+
 def serialize_verifying_key(vk):
     key_type = ECDSA_KEY_PREFIX + ECDSA_CURVE_NAME
     curve_name = ECDSA_CURVE_NAME
diff --git a/sshagent/trezor.py b/sshagent/trezor.py
index 1e01fb4..aa7003c 100644
--- a/sshagent/trezor.py
+++ b/sshagent/trezor.py
@@ -133,8 +133,9 @@ class Client(object):
 
 def message_digest(hidden, visual):
     from bitcoin import electrum_sig_hash
-    hashfunc = lambda data: formats.hashfunc(data).digest()
-    return electrum_sig_hash(hashfunc(hidden) + hashfunc(visual))
+    hidden_digest = formats.hashfunc(hidden).digest()
+    visual_digest = formats.hashfunc(visual).digest()
+    return electrum_sig_hash(hidden_digest + visual_digest)
 
 
 _identity_regexp = re.compile(''.join([