Error: Variable 'thisapp' is not declared

Hi,
I'm getting a weird error message when trying to use a custom function in the OnLoad script of a stateless form: "Variable 'thisapp' is not declared". The form is https://creator.zoho.com/ahatton/form/278/3/scriptbuilder/# and I have allowed edit access to support.

The code I'm using is (and I generated this using the script build to make sure):

prefix_str=thisapp.child_registration.getPrefix(input.child_number);

and the function declaration is:

string child_registration.getPrefix(int child_num)
{
    prefix = "";
    if (child_num == 1)
    {
        prefix = "first";
    }
    else if (child_num == 2)
    {
        prefix = "second";
    }
    else if (child_num == 3)
    {
        prefix = "third";
    }
    else if (child_num == 4)
    {
        prefix = "fourth";
    }
    else if (child_num == 5)
    {
        prefix = "fifth";
    }
    else if (child_num == 6)
    {
        prefix = "sixth";
    }
    else if (child_num == 7)
    {
        prefix = "seventh";
    }
    else if (child_num == 8)
    {
        prefix = "eighth";
    }
    return prefix;
}

The function works fine when I add it inline.