human
Solve the following python programming problem given the following code:
def find_one_or_404(self, * args, **kwargs) : 
    "Find and return a single document, or raise a 404 Not Found
        exception if no document matches the query spec. See
        :meth:`~pymongo.collection.Collection.find_one` for details.

        .. code-block:: python

            @app.route('/user/<username>')
            def user_profile(username):
                user = mongo.db.users.find_one_or_404({'_id': username})
                return render_template('user.html',
                    user=user)
        " 
    found = self.find_one (* args, ** kwargs) 
    if (kwargs is None) : 
        abort (404) 
return found 

    
    

    Fix the buggy line: if (kwargs is None) :
assistant
#Here is Python code to find and return a single document, or raise a 404 Not Found
.

if (found is None) :