HTTP POST XML到REST Singleton Service会抛出NotSupportedException

I have the following REST Host in a silverlight application. I'm getting a NotSupportedException on the EndGetResponse() call of the Save Method. In this sample I'm using the default config for a REST Singleton Service from the WCF REST Starter Kit, with the exception of some changes to the OnAddItem method to accept updates via a POST (since silverlight PUT isn't supported). I'm getting a StatusCode 200 when I make a post from Fiddler. The Service is hosted on the same site as the SilverlightPage, so I don't have any x-domain policy stuff setup. Am I missing something obvious?I have the following REST Host in a silverlight