Hiding contextual menu items

I have an NSTableView with a contextual menu, that's automatically updated using the -menuNeedsUpdate: delegate method. I want to conditionally hide some items, depending on the current state. So I tried to call -setHidden on the items on the menu in my implementation of -menuNeedsUpdate:. This does not work, the menu items are all shown, no matter whether I pass YES or NO to this method. What's going on, why doesn't it respect my settings? And is there an alternative way to do this?

thanks,
Christiaan