WCF REST service not returning DataContract object

I have a simple WCF service defined by the following ServiceContract:

[ServiceContract]
public interface IInventoryService
{
  [OperationContract]      
  Item GetItemFromBarcode(string barcode);

  [OperationContract]
  string Test(string testString);
}

With Item defined like this:

[DataContract]
public class Item
{
  [DataMember]
  public virtual int Id { get; set; }

  <Snip>
}

And the actual service implemented thusly:

public class InventoryService : IInventoryService
{
  [WebGet(UriTemplate = "/Barcode/{barcode}/Item", ResponseFormat = WebMessage开发者_如何学JAVAFormat.Json)]
  public Item GetItemFromBarcode(string barcode)
  {
     var item = (from b in repository.Query<ItemBarcode>()
             where b.BarcodeData == barcode
             select b.Item).FirstOrDefault();
     return item;

  }

  [WebGet(UriTemplate = "/Test/{testString}",ResponseFormat=WebMessageFormat.Xml)]
  public string Test(string testString)
  {
     return testString;
  }
}

With the following in the app.config for the program hosting the service:

<system.serviceModel>      
  <behaviors>
     <serviceBehaviors>
        <behavior>
           <serviceMetadata httpGetEnabled="true"/>
        </behavior>
     </serviceBehaviors>
     <endpointBehaviors>
        <behavior name="RESTFriendly">
           <webHttp />
        </behavior>
     </endpointBehaviors>
  </behaviors>
  <services>
     <service name="InventoryService">            
        <endpoint address="/Inventory" behaviorConfiguration="RESTFriendly" binding="webHttpBinding" contract="IInventoryService"/>
        <host>
           <baseAddresses>
              <add baseAddress="http://localhost:8080/"/>
           </baseAddresses>
        </host>
     </service>
  </services>
</system.serviceModel>

Now that the code dump is out of the way, the issue: I can invoke the Test method just fine with curl or Fiddler, it returns a serialized string. However, invoking the method that returns an object returns nothing. Curl spits back curl: (56) Failure when receiving data from the peer and Fiddler responds ReadResponse() failed: The server did not return a response for this request.

From what I read, this should Just Work (tm). Is there something obvious I'm missing?

---

So, it seems you can't have DataMember of an interface type, like the IList<ItemBarcode>. There go my hopes of using my NHibernate model objects as DTOs.