User Grid on System Form: SetCellEditable and SetRowEditable Don't Work?

Hi,

I have a user defined grid on a custom tab on a system form (Service Call). I am fill that grid with a DataTable. I am trying to disable certain rows but I get an error. The error is -7006 The item is not a user-defined item [66000-8]. I can call the SetCellBackColor and SetRowBackColor methods and they work fine but I really need to disable my rows. Why doesn't this work? The item is a user-defined item so the error doesn't make sense.

Thanks, David