Problems importing customers onto projects using the UPDATE_PROJECT API

I have a Contract project with no customers assigned to it and I would like to import customers onto the project using the UPDATE_PROJECT API procedure.

If I leave the option "Customer at Top Task" unchecked under the Billing Setup, then I am able to successfully import the customer onto the project using the API.  However, if I check this option I receive the error code "PA_INVALID_TASK_CUSTOMER" and the customer import fails.

I do want the customer assigned to the top task, so I would like this option checked and I would like the API to import the customer and assign the customer to this top task.  Does anyone know if this can be done through the API and if so, exactly how I would do it?