Some special characters are not generated

Hi!
First of all thanks for such a great repo.

I've got a problem with generating images with some special characters, e.g. ":" or "%", however "-" character is generating correctly. When these characters are placed with some text ("dog%") there's no problem generating them, only for solo ones.
I've tried to play with `min_nchar` parameter, but nothing helps. Looked through all issues, can't find something related to.

So, if anyone knows where should I look for I'll be very grateful for your help!