diff --git a/lib/tfw/crypto.py b/lib/tfw/crypto.py
index bdd1fad..05f1daa 100644
--- a/lib/tfw/crypto.py
+++ b/lib/tfw/crypto.py
@@ -21,6 +21,7 @@ def message_checksum(message):
 
 
 def sign_message(key, message):
+    message.pop('signature', None)
     signature = message_signature(key, message)
     message['signature'] = b64encode(signature).decode()