Personalization returning Value cannot be null. Parameter na

Hello,

I've developed a custom class project library that contains my personalization criteria similar to the example found on the website. The criteria is successfully registered, but when trying to set a criteria characteristic it throws a 500 Error "Value cannot be null. Parameter name: input" when hitting  /CriterionEditor.svc/MemberOrStaff.  Any ideas of why this is happening?

Here's the project setup:

ProjectName.Core
 -- Personalization
    -- Editor.ascx (marked as embedded resource)
    -- Evaluator.cs
    -- Resouces.cs
    -- Installer.cs

SitefinityWebApp
  -- Reference to ProjectName.Core