package-user-dir无效的功能：方便

Question

我将此行保留在.emacs文件中，以便跨计算机同步已安装的软件包：

(setq package-user-dir "~/Dropbox/Log/elpa")

这适用于计算机A，但在计算机B上，我收到Invalid function: convenience错误。这是我运行--debug-init时得到的。

Debugger entered--Lisp error: (invalid-function "convenience")
  "convenience"("vc" "git")
  byte-code("\300\301\302\303\304\305\306\307\310\311\312\"&\207" [define-package "git-commit-mode" "20140605.520" "Major mode for editing git commit messages" nil :url "https://github.com/magit/git-modes" :keywords "convenience" "vc" "git"] 11)
  load("c:/Users/username/Dropbox/Log/elpa/git-commit-mode-20140605.520/git-commit-mode-pkg" nil t)
  package-load-descriptor("~/Dropbox/Log/elpa" "git-commit-mode-20140605.520")
  package-maybe-load-descriptor("git-commit-mode" "20140605.520" "~/Dropbox/Log/elpa")
  package-load-all-descriptors()
  package-initialize()
  eval-buffer(#<buffer  *load*-681561> nil "c:/Users/username/Dropbox/Log/elisp/main.el" nil t)  ; Reading at buffer position 242
  load-with-code-conversion("c:/Users/username/Dropbox/Log/elisp/main.el" "c:/Users/username/Dropbox/Log/elisp/main.el" nil nil)
  load("c:/Users/username/Dropbox/Log/elisp/main.el" nil nil t)
  load-file("~/Dropbox/Log/elisp/main.el")
  eval-buffer(#<buffer  *load*> nil "c:/Users/username/.emacs.d/init.el" nil t)  ; Reading at buffer position 42
  load-with-code-conversion("c:/Users/username/.emacs.d/init.el" "c:/Users/username/.emacs.d/init.el" t t)
  load("c:/Users/username/.emacs.d/init" t t)
  #[0 "\205\262

我该如何解决这个问题？