Contrast this:

  $ emacs --batch --eval '(message "to stderr.")' > /dev/null
  to stderr.

With that (it produces no output):

  $ emacs --batch --eval '(princ "to stderr?" t)' > /dev/null

And yet according to the docs:

  Any Lisp program output that would normally go to the echo area,
  either using message, or using prin1, etc., with t as the stream, goes
  instead to Emacs’s standard error descriptor when in batch mode.
  -- https://www.gnu.org/software/emacs/manual/html_node/elisp/Batch-Mode.html

What am I doing wrong?
Clément.

In GNU Emacs 25.1.50.7 (x86_64-pc-linux-gnu, GTK+ Version 3.18.9)
 of 2016-07-20 built on clem-w50-mint
Repository revision: a1a0c208e3e895a6ea0942e8e5c4077faf12c7ad
Windowing system distributor 'The X.Org Foundation', version 11.0.11803000
System Description:	Linux Mint 18 Sarah