To quote steemd.py for def sign:

param string wifs: One or many wif keys to use for signing
                a transaction. If not present, the keys will be loaded
                from the wallet as defined in "missing_signatures" key
                of the transactions.

it is signed successfully, it gets the key from my wallet. It worked with no_broadcast=True.
And the error occurs in tb.broadcast()
Also, previous tests with different operations(but with active key, to be true) worked with no_broadcast=False
Anyway, what's done is done;)