SDL_ttf - some fonts show only squares

I’m trying to find a suitable (compatible with GPL) TTF font to use
in TuxPaint. (I discovered that the one I grabbed from some IceWM theme
just happened to be an Apple Mac font!!!)

A number of fonts I’ve found haven’t worked for me (at least here on my
box at work - a Debian stable box). When I try using them in TuxPaint,
or even just by running the SDL_ttf demo program ‘showfont’ on them,
I just get squares (instead of “ABC” I get “[][][]”)

Other people (well, at least one person running Debian testing) have
no problems, so I’m guessing I’m misconfigured somehow.

Only SOME fonts don’t work for me. :^/

(the last font I tested is a GPL’d font that comes with the program “yudit”…
it’s an X11 app (not SDL) and shows its own font just perfectly :^/ )