将mongoengine从0.8.7升级到0.10.9时出错

Question

在将mongoengine从0.8.7升级到0.10.9时，发现以下问题：

handle = connect(mdb_db, username = mdb_username, password = mdb_passwd, host = mdb_host, port = mdb_port) 
Traceback (most recent call last):   File "<stdin>", line 1, in <module>   File "lib/python2.7/site-packages/mongoengine/connection.py", line 224, in connect
        return get_connection(alias)   File "lib/python2.7/site-packages/mongoengine/connection.py", line 184, in get_connection
        raise ConnectionError("Cannot connect to database %s :\n%s" % (alias, e)) mongoengine.connection.ConnectionError: Cannot connect to database default : 'str' object has no attribute 'copy'

但是，仅将pymongo从2.8.1升级到2.9.5并将mongoengine保持在0.8.7不会破坏连接方法。

mongoengine文档或更改日志未涵盖此问题。

关于发生了什么问题的任何输入？另外，该如何解决？