FontLab OpenType Generate Errors

Fontlab errors when modifications are made to font


I'm having a problem making a basic modification to a font in FLS. I'm only learning FLS, so I assume I'm doing something stupid, but I can't nail down what it is.

I'm trying to modify the font Crimson, adding in a tabular oldstyle. I've already added two discretionary ligatures (no problems), and amended the OT substitution so that it correctly subs in ligatures and c2sc, and it all works great in-browser using font-feature settings (the main aim of the exercise).