I want a conditional ", Part n" on my movie format if there are likely more than 1 parts.

I've set my format up as this: /Volumes/Macintosh HD/MoviesTest/{n} ({y}){(pi == null ? '' : ", Part $pi")}
And it works fine in the GUI (see screenshot)
Screen Shot 2013-04-18 at 21.19.39.png

But in the ...