WebAPI 2 - Adding related object generates JSON error when retrieving values

Hi:

I am using WebAPI 2.1.  I have a simple class defined as follows:

 public class Statement
    {
        public Statement(int id, string name)
        {
            Id = id;
            Name= name;
        }

        //For EF
        public Statement()
        {

        }
        public int Id { get; set; }
        public string Name { get; set; }
    }
      

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When I make an http request, such as mysite.com/api/statements, it works fine and retrieves the data.  When I add the the following property to my class -
        public virtual Activity Activity { get; set; } - and make a get request, I get the following error message

Message: "An error has occurred.",
ExceptionMessage: "The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; charset=utf-8'.",
ExceptionType: "System.InvalidOperationException",
StackTrace: null,
InnerException: {
Message: "An error has occurred.",
ExceptionMessage: "Error getting value from 'Activity' on 'System.Data.Entity.DynamicProxies.Statement_6244758049322F2013F45504488584D62A3E3D45B5CDB4D16F0EA721E9E737B1'.",
ExceptionType: "Newtonsoft.Json.JsonSerializationException",.........:

Any suggestions on what I am doing wrong greatly appreciated.

RBS