'ADDTOPICKLIST' can be used only in on load actions - error message

'ADDTOPICKLIST' can be used only in on load actions - error message

Would like to be able to populate a drop down list based on value of another field.

Works fine on load action, but on update get error above.


if (input.Title  ==  "Mr")
{
    Singing_Voice:ui.add("Tenor");
    Singing_Voice:ui.add("Base");
}
else
{
    Singing_Voice:ui.add("Soprano");
    Singing_Voice:ui.add("Mezo-Soprano");
    Singing_Voice:ui.add("Alto");
}

Thanks

Tim