Inconsistent output when rendering markdown with the node-markdown module

I'm trying to write a controller in express that takes some markdown and returns some HTML for forum posts.

I get the expected behaviour at the command line:

> var md = require('node-markdown').Markdown;
undefined
> md("\n\ndon't mind me\n====")
'<h1>don\'t mind me</h1>'

But then if I receive a HTTP POST request which according to Chrome has the following form data

raw:\n\ndon't mind me\n====

I receive this response:

<p>\n\ndon't mind me\n====</p>

It's not correctly identifying the H1 tag.

Here's the code for parsing the markdown:

var md = require('node-markdown').Markdown;

var parseMarkdown = (function () {
return function (raw_md) {
    return md(raw_md);
}
}());

Here's my controller code:

app.post('/render_markdown', function (req, res) {
    res.header('Access-Control-Allow-Origin', '*');
            res.header('Content-Type', 'application/json');
    var raw = req.body['raw'];
    res.send({ html: parseMarkdown(raw)});
});

Anyone got a clue what might be going on here? I've tried fiddling with escaping and unescaping when passing the input into the Markdown function and it didn't seem to help.