Data Entities Container fields

I have a need of using unmapped fields on a data entity of type container. I know that there are 2 ways in using unmapped fields, either by setting the isComputed property to yes, or no.

I have created a new unmapped container field on the FMCustomer extension entity for testing. In the post load method, I am setting a value for this container, as below:

    public void postLoad()
    {
        super();
        this.Age = this.calculateAge();  //integer
        this.PaymentInformation = ["test1", "test2"];  //container
    }

As you can see, there is another unmapped field of type int, which is being calculated and loaded correctly. However, the container is either loaded as null, or a system null exception is thrown.

Any ideas on how to solve this? I know that this is a new addition, possibly on update 2, and maybe it has not been implemented fully yet.

Thanks,

MAtthew