WCF XMLSerializerFormat: I keep getting casting errors if I use it!

Question

I keep getting an error when I have an interface with more than one method tagged with the XMLSerializerFormatAttribute and I'm not sure why...

[ServiceContract(Namespace = "http://www.mysite.com/Services/Foo", ConfigurationName = "IFoo")]
public interface IFoo
{
    [OperationContract(Action = "http://www.mysite.com/Services/Foo/SelectBarOne", ReplyAction = "*")]
    [XmlSerializerFormat(SupportFaults = true)]
    BarOneResponse SelectBarOne(BarOneRequest request);

    [OperationContract(Action = "http://www.mysite.com/Services/Foo/SelectBarTwo", ReplyAction = "*")]
    [XmlSerializerFormat(SupportFaults = true)]
    BarTwoResponse SelectBarTwo(BarTwoRequest request);
}

[MessageContract(IsWrapped = false)]
public sealed class BarOneRequest
{
    public BarOneRequest() { }
}

[MessageContract(IsWrapped = false)]
[ServiceKnownType(typeof(BarOne))]
public sealed class BarOneResponse
{
    public BarOneResponse()
    {
        Init();
    }

    public BarOneResponse(BarOne[] entities = null)
    {
        Init(entities);
    }

    private void Init(BarOne[] entities = null)
    {
        Entities = entities;
    }

    [MessageBodyMember(Namespace = "", Order = 0)]
    public BarOne[] Entities { get; private set; }
}

[MessageContract(IsWrapped = false)]
[ServiceKnownType(typeof(BarTwo))]
public sealed class BarTwoRequest
{
    public BarTwoRequest() { }
}

[MessageContract(IsWrapped = false)]
[ServiceKnownType(typeof(BarTwo))]
public sealed class BarTwoResponse
{
    public BarTwoResponse()
    {
        Init();
    }

    public BarTwoResponse(BarTwo[] entities = null)
    {
        Init(entities);
    }

    private void Init(BarTwo[] entities = null)
    {
        Entities = entities;
    }

    [MessageBodyMember(Namespace = "", Order = 0)]
    public BarTwo[] Entities { get; private set; }
}

My MS Service Tracer gives me the following error: System.InvalidCastException: Unable to cast object of type 'BarTwo[]' to type 'BarOne[]'. I really think it has to do with one of my attribute tags not being proper, but I can't figure out the correct combination.