Photoshop saying "0 files selected" when creating contact sheet via Bridge

I'm trying to create a contact sheet so select my images in Bridge then go to Tools > Photoshop > Contact Sheet II (as this article told me to do). But then Photoshop seems to think no images have been selected. Doesn't matter whether I select 1 or 100, same thing comes up. I've never used Bridge before - am I missing something?