I am using a third party web service that offers the following calls and responses

http://api.athirdparty.com/rest/foo?apikey=1234

<response>
  <foo>this is a foo</foo>
</response>

and

http://api.athirdparty.com/rest/bar?apikey=1234

<response>
  <bar>this is a bar</bar>
</response>

This is the contract and supporting types I wrote

[ServiceContract]
[XmlSerializerFormat]
public interface IFooBarService
{
    [OperationContract]
    [WebGet(
        BodyStyle = WebMessageBodyStyle.Bare,
        ResponseFormat = WebMessageFormat.Xml,
        UriTemplate = "foo?key={apikey}")]
    FooResponse GetFoo(string apikey);

    [OperationContract]
    [WebGet(
        BodyStyle = WebMessageBodyStyle.Bare,
        ResponseFormat = WebMessageFormat.Xml,
        UriTemplate = "bar?key={apikey}")]
    BarResponse GetBar(string apikey);
}

[XmlRoot("response")]
public class FooResponse
{
    [XmlElement("foo")]
    public string Foo { get; set; }
}

[XmlRoot("response")]
public class BarResponse
{
    [XmlElement("bar")]
    public string Bar { get; set; }
}

and then my client looks like this

static void Main(string[] args)
{
    using (WebChannelFactory<IFooBarService> cf = new WebChannelFactory<IFooBarService>("thirdparty"))
    {
        var channel = cf.CreateChannel();
        FooResponse result = channel.GetFoo("1234");
    }
}

When I run this I get the following exception

Unable to deserialize XML body with root name 'response' and root namespace '' (for operation 'GetFoo' and contract ('IFooBarService', 'http://tempuri.org/')) using XmlSerializer. Ensure that the type corresponding to the XML is added to the known types collection of the service.

If I comment out the GetBar operation from IFooBarService, it works fine. I know I'm missing an important concept here - just don't know quite what to look for. What is the proper way to construct my contract types, so that they can be properly deserialized?