Won't let me add new item to a Pick List

Won't let me add new item to a Pick List

In attempting to add a new item to a Pick List I: 

Select Setup, then Fields under the Customization menu, then select my Potentials module, then select Edit on my Stage field.  (I've done this many times before to successfully add a new value to my Pick List for the Stage field.)

But now rather than take me to the screen where I can add a new value it takes me to the Stage-Probability Mapping screen.  If I leave this screen by entering either Cancel or Save it takes me back to the List of Fields page.  

I am able to add items to the Pick List for other fields, but not to my Stage field.  

Seems like a program bug.  Any ideas?