[luatex] font.fonts error "error: attempt to index a thread value"

[Patrick Gundlach]

Hi,

I know this is a very vague question:

Sometimes (while going through the node list on a pre_linebreak_filter  
on a glue item) I get an error when using font.fonts[x] or font.getfont 
(x). The fonts are defined by using font.define(f) (f is a table that  
is constructed from the fontloader lib similar to the page on the  
bluwiki). The error I get is:

error: attempt to index a thread value

Does anyone by chance have an idea where to start looking? I know I  
have to make a minimal example, but perhaps somebody can tell me from  
the error message.

Patrick