File Mode Specification Error: (void-function nil) after opening python file

Question

every time I try to open a python file I get the following message on the minibuffer:

File Mode Specification Error: (void-function nil)

I turned on the debug mode but I honestly don't understand what is going on or how to narrow down the error:

Debugger entered--Lisp error: (void-function nil)
nil()
run-hooks(change-major-mode-after-body-hook prog-mode-hook python-mode-hook)
apply(run-hooks (change-major-mode-after-body-hook prog-mode-hook python-mode-hook))
run-mode-hooks(python-mode-hook)
python-mode()
set-auto-mode-0(python-mode nil)
set-auto-mode()
normal-mode(t)
after-find-file(nil t)
find-file-noselect-1(#<buffer flasktest.py> "~/tmp/flasktest.py" nil nil "~/data/tmp/flasktest.py" (8388619 2052))
find-file-noselect("/home/srp/tmp/flasktest.py" nil nil)
ido-file-internal(raise-frame)
ido-find-file()
call-interactively(ido-find-file nil nil)
command-execute(ido-find-file)

I went through my init.el but didn't find any suspicious with respect to python mode. Here are the relevant lines:

;; Package Management
(require 'package)
(add-to-list 'package-archives
         '("melpa" . "https://melpa.org/packages/") t)
(package-initialize)
...
...
;; PYTHON IDE
;; Install rope, flake8, importmagic, autopep8 via pip
(elpy-enable)
(require 'py-autopep8)
(add-hook 'elpy-mode-hook 'py-autopep8-enable-on-save)
(autoload 'pylint "pylint")
(add-hook 'python-mode-hook 'pylint-add-menu-items)
(add-hook 'python-mode-hook 'pylint-add-key-bindings)
(add-hook 'python-mode-hook (highlight-indentation-mode 0))

Does anyone know how to fix that error?

Answer 1

This is your problem (at least one problem):

(add-hook 'python-mode-hook (highlight-indentation-mode 0))

What does (highlight-indentation-mode 0) return? (My guess is nil, not a function.)

What it returns is added as an element of python-mode-hook. Then that thing, whatever it is, is invoked as a function. In this case, Emacs tries to invoke nil as a function. No can do.