SerializationException with WCF services

I have a WCF service set up as following:

    [ServiceContract]
    public interface IObjectPresentationService
    {
        [OperationContract(Name="GetFooter")]
        [WebInvoke(Method = "POST",
      BodyStyle = WebMessageBodyStyle.WrappedRequest,
      ResponseFormat = WebMessageFormat.Json)] 
        string GetFooter();
     }

When I use Add Service Reference on the client side, the following code is generated:

[System.ServiceModel.OperationContractAttribute(Action="http://tempuri.org/IObjectPresentationService/GetFooter", ReplyAction="http://tempuri.org/IObjectPresentationService/GetFooterResponse")]
        [WebInvoke(Method = "POST", BodyStyle = WebMessageBodyStyle.WrappedRequest, ResponseFormat=WebMessageFormat.Json)]
        string GetFooter();

However, when I try to execute GetFooter(), I get the following exception:

SerializationException was unhandled OperationFormatter encountered an invalid Message body. Expected to find an attribute with name 'type' and value 'object'. Found value 'string'.

Using Fiddler I can see that the service indeed returns a plain string, complete with quotemarks at the begining and end (wh开发者_StackOverflow社区ich should make it valid json?).

Could this be an configuration issue?