That's a response from Blockchain(dot)com's API if you've sent multiple queries in just a short while or your IP is above their limit.

Pywallet is using that as its main API endpoint and since there's no error response and Pywallet doesn't know how to handle such value, it took it (rate limited) as your balance.
Use manual query like what Loyce suggested or change the API endpoint in the code to another balance check API.