Make text field visible based on drop down value

I have a drop down list which listed 2 items (Old ID and Army ID).
I've also created text field for each for the above item.
Currently, I make all the text field invisible and want to make them visible
only based on what user choose in the drop down list.

if (DropDownList3.rawValue == "Old ID")
{
xfa.form.form1.old_nric_no.presence = "Visible" ;
}
else if (DropDownList3.rawValue == "Army")
{
xfa.form.form1.Army.presence = "Visible" ;
}
end if

What could be wrong with my script above. When i choose Old ID, the required
field doesn't appear.