Why does notify-send fail with a "GLib-GObject-CRITICAL" error on Red Hat Enterprise Linux 6?

Issue

• Why am I unable to send a notification to a remote RHEL desktop using notify-send?

• notify-send fails with a GLib-GObject-CRITICAL error.

  $ notify-send -t 3000 "aaa"
  
  (notify-send:103211): GLib-GObject-CRITICAL **: g_object_unref: assertion `G_IS_OBJECT (object)' failed