[luatex] Debugging the error 'Expected a string for (S), not: nil'

[Patrick Gundlach]

> I get the error
> 
> "Expected a string for (S), not: nil"
> 
> which seems to be related to callbacks. How can I debug, what is wrong with my code? Is something wrong with my callback re-defintions? 


More information: it seems to happen while executing the following code:

        tex.box[666] = n
        tex.shipout(666)


(n is a vlist node)

and this is the code related to printing the page number:


function print_page_number()
  texio.write(string.format("> Shipout page %d\n",current_pagenumber))
end
callback.register("start_page_number",print_page_number)
callback.register("stop_page_number",false)



Patrick