steem-python: Claiming Reward Balance gives MissingKeyError

I was trying to use the claim_reward_balance(). It worked for sometime, but two days ago it stops working.

The code:

from steem import Steem
from steem.commit import Commit

c =  Commit(steem=Steem(keys=[POST_KEY, ACTIVE_KEY]))

 c.claim_reward_balance()

MissingKeyError                           Traceback (most recent call last)
<ipython-input-6-537da9054b42> in <module>()
----> 1 c.claim_reward_balance()

/usr/local/lib/python3.6/dist-packages/steem-1.0.0-py3.6.egg/steem/commit.py in claim_reward_balance(self, reward_steem, reward_sbd, reward_vests, account)
    912                 "reward_vests": reward_vests,
    913             })
--> 914         return self.finalizeOp(op, account, "posting")
    915
    916     def delegate_vesting_shares(self, to_account, vesting_shares,

/usr/local/lib/python3.6/dist-packages/steem-1.0.0-py3.6.egg/steem/commit.py in finalizeOp(self, ops, account, permission)
    130         else:
    131             tx.appendSigner(account, permission)
--> 132             tx.sign()
    133
    134         return tx.broadcast()

/usr/local/lib/python3.6/dist-packages/steem-1.0.0-py3.6.egg/steem/transactionbuilder.py in sign(self)
    119
    120         if not any(self.wifs):
--> 121             raise MissingKeyError
    122
    123         signedtx.sign(self.wifs, chain=self.steemd.chain_params)

MissingKeyError: