Why can't I see the character _ while using Nano on Yakuake?

I use Gnu Nano on Yakuake to write a LaTeX-document. Then yesterday I found that I can't see the character _ on the Nano. I know that the characters are there as LaTeX renders the tex-file correctly. Is that a bug in Yakuake or Nano or is there some settings that is wrong in my system?

Here is a demo. The code that looks like \lim {r\to\infty} is really \lim_{r\to\infty}