[luatex] img.scan segfault

[Andreas Matthias]

Using img.scan() in a tex file with \directlua works fine:

\directlua{
  a = img.scan({filename='foo.jpg'}) 
}

But it segfaults if I use img.scan in a pure lua file and
run it with --onlylua:

  a = img.scan({filename='foo.jpg'}) 

I have only this one line in the lua file. Do I need to import
additional modules?

Ciao
Andreas