The current version of lualatex distributed with TeX Live (as installed today via TeX Live's install-tl, or via Arch Linux's packages) replaces em dashes with hyphens if they're not separated by any space.
It looks as if I was wrong. It is not a luaotfload problem, but of the generic fontloader. It doesn't look as if this will change, so you have the options 1. Add \automatichyphenmode=1 This will avoid the error at the cost that there will be no longer a possible line break point after the em dash (perhaps we will make this the default. It will also prevent line breaks after the hyphen e.g. for $p$-adic and if the hyphen is at the begin of the word, something that is good for some languages.) 2. Input the emdash directly: —. I wouldn't like this much, as I can type --- much faster than —, also --- is more visible, but it could be that it is your best option. -- Mit freundlichen Grüßen Ulrike Fischer