[O] Org-mode won't let me export via markdown

[Avery Chan]

First, thanks to everyone who contributes to org-mode. I find it very
useful.

When I select export as markdown, I get an error:

  Quit [2 times] not: Wrong type argument: stringp, nil

I've posted a backtrace in the debugger to here:
http://pastebin.com/UAAund8M Emacs: 24.3.50.1 Org-mode: 8.0.3

I've edited my .emacs appropriately to load the md exporter
backend. As a sidenote, it would be nice to put into the documentation
that there needs to be some configuration to get markdown export to
work. I was confused for a while why I didn't see the markdown
options.