我已经使用代码将工作项添加到Visual Studio Online中，使用v1.0 Preview 2 API进行工作项跟踪一个多月了，现在似乎已经破坏了我对API的使用它不再是预览版。在更新运行之前发送的JSON相同。发送的数据是：
[{"op":"add","path":"/fields/System.Title","value":"dev task"},{"op":"add","path":"/fields/System.IterationPath","value":"Valid Iteration Path"},{"op":"add","path":"/fields/Microsoft.VSTS.Common.Activity","value":"Development"},{"op":"add","path":"/relations/-","value":{"rel":"System.LinkTypes.Hierarchy-Reverse","url":"https://myurl.visualstudio.com/DefaultCollection/_apis/wit/workItems/2722","attributes":{"comments":"Added sub task"}}},{"op":"add","path":"/fields/System.AssignedTo","value":"Valid User"}]
我现在收到的回复是：
{"$id":"1","innerException":null,"message":"Nullable object must have a value.","typeName":"System.InvalidOperationException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089","typeKey":"InvalidOperationException","errorCode":0,"eventId":0}
我是否需要一些我不知道的新内容？