Hi,

According to the documentation of call-process-region,
  If START is nil, that means to use the entire buffer contents; END is
ignored.

But when I run:
(call-process-region nil nil "/bin/cat" t (current-buffer))
I get:
Debugger entered--Lisp error: (wrong-type-argument integer-or-marker-p nil)
  call-process-region(nil nil "/bin/cat" t #<buffer *scratch*>)
  eval((call-process-region nil nil "/bin/cat" t (current-buffer)) nil)
  elisp--eval-last-sexp(t)
  eval-last-sexp(t)
  eval-print-last-sexp(nil)
  funcall-interactively(eval-print-last-sexp nil)
  call-interactively(eval-print-last-sexp nil nil)
  command-execute(eval-print-last-sexp)

If, instead, I run:
(call-process-region (point-min) (point-max) "/bin/cat" t (current-buffer))

It works.

Am I interpreting the documentation wrong?

Emacs version:
GNU Emacs 26.3 (build 1, x86_64-apple-darwin18.2.0, NS appkit-1671.20
Version 10.14.3 (Build 18D109)) of 2019-09-02


Thank you