AWS CLI - All commands return Unknown output type: [None]

Question

All of my aws-cli commands returned

Unknown output type: [None]

I checked my configuration

$ aws configure

appeared normal but i was unable to edit my 'Default output format'

I ran my aws-cli command with --debug and saw

MainThread - awscli.clidriver - DEBUG - Exception caught in main()
Traceback (most recent call last):
  File "/Library/Python/2.7/site-packages/awscli/clidriver.py", line 208, in main
    return command_table[parsed_args.command](remaining, parsed_args)
  File "/Library/Python/2.7/site-packages/awscli/clidriver.py", line 345, in __call__
    return command_table[parsed_args.operation](remaining, parsed_globals)
  File "/Library/Python/2.7/site-packages/awscli/clidriver.py", line 517, in __call__
    call_parameters, parsed_globals)
  File "/Library/Python/2.7/site-packages/awscli/clidriver.py", line 638, in invoke
    self._display_response(operation_name, response, parsed_globals)
  File "/Library/Python/2.7/site-packages/awscli/clidriver.py", line 657, in _display_response
    formatter = get_formatter(output, parsed_globals)
  File "/Library/Python/2.7/site-packages/awscli/formatter.py", line 272, in get_formatter
    raise ValueError("Unknown output type: %s" % format_type)
ValueError: Unknown output type: [None]

Answer 1

$aws configure
press Enters
see the "Default output format [None]:" 
input one of "json, text or table "(all in lower case) 
after that rerun your command. 

Answer 2

Run the command "aws configure", then check if the word "JSON" in "Default output format [JSON]:" is in upper case or lower case? If it's in upper case then running any aws command shows "Unknown ouput type : JSON".
Or alternately open the file C:\Users\<user>\.aws\config file and check the entry "output = json". If the word json is in upper case then running any aws command shows "Unknown ouput type : JSON".

Solution:
Replace the upper case JSON with lower case json.