Traceback (most recent call last):


 "<stdin>", line 1, in <module>


 "mongomock/collection.py", line 1586, in aggregate

out_collection = _extend_collection(out_collection, field, value)


 "mongomock/collection.py", line 1464, in _extend_collection

doc[field] = _parse_expression(expression.copy(), doc)
AttributeError: 'int' object has no attribute 'copy'

Traceback (most recent call last):


 "<string>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'label'
