Using TTF fonts shows rectangles (for non existant characters in fonts)

I used the ms core fonts package on Debian to give some fonts for textimage, and found that sometimes it shows one of those rectangles for characters that do not exist in the font. I don't know if there is any way around this but I had to resort to the default font because of this (I could try to eliminate all the fonts that might have this problem but I don't really like that idea). It doesn't help to switch on Caps only, there are apparantly still fonts with a problem there.