urllib2.HTTPError: HTTP Error 422

Question

I am trying to utilize https://poloniex.com/support/api/

Python wrapper by oipminer:

self.p.api_query('buy', {"currencyPair":'BTC_STEEM', "rate":str(0.00020410), "amount":str(0.0001)})

But I always get 422 error:

urllib2.HTTPError: HTTP Error 422

What do I do?

UPD:

It is no always gives an error, sometimes something like this can happen(as you can see it does not give a 422error)

 currencyPair:BTC_ETH, rate: 0.04816195, amount: 0.103816394477 first trade:{u'error': u'Not enough BTC.'}