Custom Workflow in Sandbox not getting InArgument

Here is the InArgument:

        [RequiredArgument]
        [Input("Email")]
        [ReferenceTarget("email")]
        public InArgument<EntityReference> Email { get; private set; }

Here is the code that includes the error:

    public class WF : CodeActivity
    {

        protected override void Execute(CodeActivityContext executionContext)
        {
            EntityReference emailApplication = this.Email.Get(executionContext);

When not ran in Sandbox mode, it works correctly. The Email is set. When I run in Sandbox mode, this.Email = null and an Exception is thrown. Why can this be? We have the newest rollup on our crm server, and the code is running on .NET Framework 4 platform update 3.