Dear list, Running the following one-liner: \directlua{callback.register('ligaturing', node.ligaturing)}ffi\bye results in: ------------------------------------------- This is LuaTeX, Version 1.09.0 (TeX Live 2019/dev/Debian) restricted system commands enabled. **\directlua{callback.register('ligaturing', node.ligaturing)}ffi\bye ! error: (linebreak): invalid list tail, probably missing glue ! ==> Fatal error occurred, no output PDF file produced! ------------------------------------------- The problem seems to be that, when restoring the original ‘tail of the list’ after applying the ligaturing callback (in this case consisting of the parfillskip glue), it is appended to the tail of the /original/ list-to-be-ligatured. The latter, of course, has been lost if the paragraph ends in a ligature. According to the manual, the first node of the list passed to the callback is guaranteed not to be a glyph_node. Should the same not hold for the last one, too? The manual states that it ‘normally’ is a glue, but that is clearly not the case here. Best, Esger