Error while adding a new child to a workitem

```
client = RTCClient(server, username = username, password = password, ends_with_jazz = True)
workitem = client.getWorkitem(133350)

workitem.addChild(133451)
```

the code works fine until getting a workitem, and when I try to add a child to it, I get the below error.

`'NoneType' object is not iterable`

I can assure that the workitem I am willing to add as a child already exists.