human
Solve the following python programming problem given the following code:
def ListCommonPrefixes(self, delimiter, callback, prefix = None, marker = None, maxkeys = None) : 
    'List files in a S3 bucket.' 
    assert (delimiter is not None), 'delimiter arg is required on ListCommonPrefixes' 
    def _OnCompletedGet(response) : 
        if response.error : 
            raise response.error 
ns = '{http://s3.amazonaws.com/doc/2006-03-01/}' 
        common_prefix_element = ('%sCommonPrefixes' % ns) 
        prefix_element = ('%sPrefix' % ns) 
        item_element = ('%sContents' % ns) 
        key_element = ('%sKey' % ns) 
        bucket_list = ElementTree.XML (response.body) 
        prefixes = [item.find (prefix_element).text for item in bucket_list.iter (common_prefix_element)] 
        items = [item.find (key_element).text for item in bucket_list.iter (item_element)] 
        callback ((prefixes, items)) 
params = dict () 
    if prefix : 
        params ['prefix'] = prefix 
if marker : 
        params ['marker'] = marker 
if (maxkeys is not None) : 
        params ['max-keys'] = maxkeys 
params ['delimiter'] = delimiter 
    self._async_s3_conn.make_request ('GET', bucket = delimiter._bucket_name, key = '', params = params, callback = _OnCompletedGet) 

    
    

    Fix the buggy line: self._async_s3_conn.make_request ('GET', bucket = delimiter._bucket_name, key = '', params = params, callback = _OnCompletedGet)
assistant
#Here is Python code to find (prefix_element).

self._async_s3_conn.make_request ('GET', bucket = self._bucket_name, key = '', params = params, callback = _OnCompletedGet)