disappearing character pair in InDesign

[William]

Are the letters by any chance f and i or f and lowercase l?

If so, it could possibly be the same problem as was discussed in this thread?

Letters disappear

I just created a font that is working perfectly except…whenever I type a lower case “f” then an “i” in a row…they both disappear. I thought it might be a kerning issue, but I redid that with no success… Any ideas?

William Overington