underbar and greek letters

[Alexander Sklar]

Hi all, I'm having trouble getting the underbar frame decoration to work
with greek letters.

I want to write this:

 

\underbar{\mu}

 

But I get compilation errors. Is this a LaTeX issue? How can I avoid it? I
don't really want to use underline instead of underbar because then the
vertical alignment gets all weird.

Thanks!

Alex