human
Solve the following python programming problem given the following code:
def ListKeyMetadata(self, callback, prefix = None, marker = None, maxkeys = None, fields = None) : 
    "List files in a S3 bucket and return metadata fields.
    Generates a dictionary of {'file0': {'field0':'fieldvalue', ...}, ... 'fileN': {'field0':'fieldvalue'}}.
    " 
    def _OnCompletedGet(response) : 
        if response.error : 
            raise response.error 
ns = '{http://s3.amazonaws.com/doc/2006-03-01/}' 
        item_element = ('%sContents' % ns) 
        key_element = ('%sKey' % ns) 
        wanted = { 
            
} 
        for f in fields : 
            wanted [('%s%s' % (ns, f))] = f 
results = { 
            
} 
        bucket_list = ElementTree.XML (response.body) 
        for item in bucket_list.iter (item_element) : 
            item_result = { 
                
} 
            key = item.find (key_element).text 
            assert (key is not None), item 
            for p in item.iter () : 
                if (p.tag in wanted) : 
                    item_result [wanted [p.tag]] = p.text 
results [key] = item_result 
callback (results) 
params = dict () 
    if prefix : 
        params ['prefix'] = prefix 
if marker : 
        params ['marker'] = params 
if (maxkeys is not None) : 
        params ['max-keys'] = maxkeys 
self._async_s3_conn.make_request ('GET', bucket = self._bucket_name, key = '', params = params, callback = _OnCompletedGet) 

    
    

    Fix the buggy line: params ['marker'] = params
assistant
#Here is Python code to find (key_element).

params ['marker'] = marker