Input leads to "undefined" in output

[ianrose14]

The following input leads to ouput containing the word "undefined":

foo
> foo

> foo

For clarity, here is the same input as a gist: https://gist.github.com/ianrose14/5879793

Here is the actual HTML that is generated:

<p>foo</p>

<blockquote><p>fooundefined&gt; foo</p></blockquote></div>