error: company-backends void:

;; nixos:
(require 'company)
(global-set-key (kbd "C-c C-S-n") 'helm-nixos-options)
(global-set-key (kbd "C-c C-u") 'nix-update-fetch)
(global-pretty-sha-path-mode)
(add-to-list 'company-backends 'company-cabal)
(add-hook 'after-init-hook 'global-company-mode)
(add-to-list 'company-backends 'company-nixos-options)
;; flycheck:
(setq flycheck-command-wrapper-function
        (lambda (command) (apply 'nix-shell-command (nix-current-sandbox) command))
      flycheck-executable-find
        (lambda (cmd) (nix-executable-find (nix-current-sandbox) cmd)))
;; haskell mode:
(setq haskell-process-wrapper-function
        (lambda (args) (apply 'nix-shell-command (nix-current-sandbox) args)))
;; end nixos