Visual Studio Team Service REST API error when creating a new iteration

I am having some trouble creating an iteration using the REST API for VSTS.

I have tried the simple example code, which is as follows (I have replace the instance and project values with my own):

https://fabrikam.visualstudio.com/DefaultCollection/Fabrikam-Fiber/_apis/work/teamsettings/iterations?api-version=v2.0-preview

The payload is as follows (I create a new GUID each time):

"{\"id\":\"a589a806-bf11-4d4f-a031-c19813331553\"}"

However, when I run this I get a BadRequest error from the server with the following error:

{"$id":"1","innerException":null,"message":"Value cannot be null.\r\nParameter name: iteration","typeName":"System.ArgumentNullException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089","typeKey":"ArgumentNullException","errorCode":0,"eventId":0}

I am able to create user stories and task without any problem.

Any ideas?

Darren.