Macro %so has illegal name - why?

Just tried the following in an rpm spec file

%define so .so

When I tried building with this (rpmbuild -bb), I got

error: Macro %so has illegal name (%define)

Why? I mean, obviously rpmbuild didn't like my macro name, so I can just 
change it to something else, but what exactly is wrong with it? What are 
the rules? I this documented anywhere?

- Toralf