Hi Hans,

this bug was reported a while ago by user Thérèse on
tex.stackexchange.com [1]. The font “Eadui.ttf” [2] causes the
fontloader to access a nil value because the “Anchor-1” table is
sparse (nil at index 3). Test file for Context:

    https://bitbucket.org/phg/lua-la-tex-tests/src/tip/cnt-eadui.tex

Patch attached, though I suspect it covers only the symptoms.

Philipp

[1] http://tex.stackexchange.com/q/195481/14066
[2] Download at http://openfontlibrary.org/font/eadui