How to fix 'Cannot convert MWArray to requested type"

I'm starting to set up a WCF Server for a Matlab application, and would like to receive a struct back using the Microsoft WCF Test Client. I need help passing a Matlab struct back successfully.

I'm following instructions provided by MATHWORKS.

I am using Windows 10, VS 2017, Community edition. I have been able to successfully communicate across WCF using a simple string reply. I can pass data into Matlab, and generate a simple graph. However, when I try using the Matlab return through a struct, I can not seem to get past the error described below.

Matlab Function:

function y = callMATLABfunction( x )
    clearvars -except x; close all;
    if nargin == 1 && ~isempty(x)
        if isnumeric(x)

       [...]

           y.Completion = 1;
           y.Result = 'FirstCompletion';
           y.Message = 'Success';

        else

        [...]          

           y.Completion = 1;
           y.Result = 'SecondCompletion';
           y.Message = 'Success';

        end
    else
        y.Completion = 0;
        y.Result = 'Error';
        y.Message = 'Please supply one input variable';
    end
end

IMatlabInterface.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using MathWorks.MATLAB.NET.Arrays;
using MathWorks.MATLAB.NET.Utility;
using System.Text;
using System.Threading.Tasks;

namespace MatlabSvc
{
    [ServiceContract]
    public interface IMatlabInterface
    {
        [OperationContract(Name = "callMATLABfunction_1")]
        y callMATLABfunction(System.Double x);
        [OperationContract(Name = "callMATLABfunction_2")]
        y callMATLABfunction(System.String x);
    }
    [KnownType(typeof(y))]
    [DataContract]
    public class y
    {
        [DataMember]
        public double Completion { get; set; }
        [DataMember]
        public string Result { get; set; }
        [DataMember]
        public string Message { get; set; }
    }
}

I expected to see the return struct in Microsoft WCF Test Client. Instead, I get:

Cannot convert MWArray to requested type

Server stack trace: at System.ServiceModel.Channels.ServiceChannel.ThrowIfFaultUnderstood(Message reply, MessageFault fault, String action, MessageVersion version, FaultConverter faultConverter) at System.ServiceModel.Channels.ServiceChannel.HandleReply(ProxyOperationRuntime operation, ProxyRpc& rpc) at System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs, TimeSpan timeout) at System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(IMethodCallMessage methodCall, ProxyOperationRuntime operation) at System.ServiceModel.Channels.ServiceChannelProxy.Invoke(IMessage message)

Exception rethrown at [0]: at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg) at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type) at IMatlabInterface.callMATLABfunction_1(Double x) at MatlabInterfaceClient.callMATLABfunction_1(Double x)

Is it possible to pass a struct back through to C# using type safe interface?