Traceback (most recent call last):
  File "./data_analysis.py", line 330, in <module>
    my_main()
  File "./data_analysis.py", line 307, in my_main
    (cuisine, ratio_cuisine) = most_popular_cuisine(db)
  File "./data_analysis.py", line 30, in most_popular_cuisine
    res1 = list(db.restaurants.aggregate(pipeline1))
  File "C:\Python27\lib\site-packages\pymongo\collection.py", line 1922, in aggregate
    collation=collation)
  File "C:\Python27\lib\site-packages\pymongo\collection.py", line 232, in _command
    collation=collation)
  File "C:\Python27\lib\site-packages\pymongo\pool.py", line 477, in command
    collation=collation)
  File "C:\Python27\lib\site-packages\pymongo\network.py", line 116, in command
    parse_write_concern_error=parse_write_concern_error)
  File "C:\Python27\lib\site-packages\pymongo\helpers.py", line 210, in _check_command_response
    raise OperationFailure(msg % errmsg, code, response)
pymongo.errors.OperationFailure: exception: pipeline element 0 is not an object