"void-variable ess-r-customize-alist" when running magit-status in R package dir

[DarwinAwardWinner]

ESS is causing me some problems when running magit-status in a directory with an R package:

Debugger entered--Lisp error: (void-variable ess-r-customize-alist)
  #f(compiled-function (var) #<bytecode -0x16f40f72fd077de5>)(ess-dialect)
  mapc(#f(compiled-function (var) #<bytecode -0x16f40f72fd077de5>) (ess-dialect ess-setwd-command ess-getwd-command ess-quit-function inferior-ess-reload-function))
  ess-r-package-mode(1)
  #f(compiled-function () #<bytecode 0x1f799f2b7e20694c>)()
  apply(#f(compiled-function () #<bytecode 0x1f799f2b7e20694c>) nil)
  ess-r-package-auto-activate()
  run-hooks(after-change-major-mode-hook)
  run-mode-hooks(magit-status-mode-hook)
  magit-status-mode()
  magit-setup-buffer-internal(magit-status-mode nil ((magit-buffer-diff-args ("--no-ext-diff")) (magit-buffer-diff-files nil) (magit-buffer-log-args ("-n256" "--decorate")) (magit-buffer-log-files nil)))
  magit-status-setup-buffer("~/Projects/TIM/TIM/")
  magit-status(nil ((9 . 5) (("/Users/thompr21/Projects/TIM/TIM/" . config) . #<hash-table equal 24/24 0x41ed95a9 ...>) (("/Users/thompr21/Projects/TIM/TIM/" . magit-toplevel) . "/Users/thompr21/Projects/TIM/TIM/") (("~/Projects/TIM/TIM/" . magit-toplevel) . "/Users/thompr21/Projects/TIM/TIM/") (("/Users/thompr21/Projects/TIM/TIM/" "rev-parse" "--show-cdup") . "") (("/Users/thompr21/Projects/TIM/TIM/" "rev-parse" "--show-toplevel") . "/Users/thompr21/Projects/TIM/TIM")))
  funcall-interactively(magit-status nil ((9 . 5) (("/Users/thompr21/Projects/TIM/TIM/" . config) . #<hash-table equal 24/24 0x41ed95a9 ...>) (("/Users/thompr21/Projects/TIM/TIM/" . magit-toplevel) . "/Users/thompr21/Projects/TIM/TIM/") (("~/Projects/TIM/TIM/" . magit-toplevel) . "/Users/thompr21/Projects/TIM/TIM/") (("/Users/thompr21/Projects/TIM/TIM/" "rev-parse" "--show-cdup") . "") (("/Users/thompr21/Projects/TIM/TIM/" "rev-parse" "--show-toplevel") . "/Users/thompr21/Projects/TIM/TIM")))
  #<subr call-interactively>(magit-status nil nil)
  apply(#<subr call-interactively> magit-status (nil nil))
  call-interactively@ido-cr+-record-current-command(#<subr call-interactively> magit-status nil nil)
  apply(call-interactively@ido-cr+-record-current-command #<subr call-interactively> (magit-status nil nil))
  call-interactively(magit-status nil nil)
  command-execute(magit-status)

Running M-x load-library ess-rd fixes the problem, which makes sense since this is where ess-r-customize-alist is defined.

As for the source of the problem, I see the following line in ess-r-package.el:

;; Silence the byte compiler, OK because this file is only loaded by
;; ess-r-mode and has no autoloads.
(defvar ess-r-customize-alist)

However, this comment makes no sense, because ess-r-mode.el does not define ess-r-customize-alist, nor does it load ess-rd.el. The obvious solution seems to be to delete this defvar and replace it with (require 'ess-rd), but I don't know if that would cause some other problem.