reftex-complete failed with (wrong-type-argument stringp nil)

Question

In org-mode, in a file with a \bibliography line, C-c [ failed. With (setq debug-on-error t), the error message was

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  re-search-forward(nil 10001 t)
  tex-main-file()
  reftex-TeX-master-file()
  reftex-tie-multifile-symbols()
  reftex-access-scan-info(nil)
  reftex-citation()
  funcall-interactively(reftex-citation)
  call-interactively(reftex-citation nil nil)
  command-execute(reftex-citation)

It has worked intermittently before. How to get it working again?

Answer 1

The system lacked the auctex package. tex-main-file referred to the non-existing file tex-mode.el.

Install auctex to fix this. F.ex. for Ubuntu

sudo apt install auctex

fixed the problem.