FVWM: WindowList in Popup?

I would like to use WindowList as a Popup submenu, but I'm having trouble getting it to work.  If I do:

        AddToMenu foo "Window List" WindowList

it works as expected, popping up the window list after I click on the label.  I'd prefer that it pop up the list when I hover as Popup does, but when I set:

        AddToMenu foo "Window List" Popup WindowList

I get the label and the Popup arrow, but it does not pop up the Window List.

Does anyone know of a way to use the WindowList command as a Popup submenu?

Thanks,
Mark