Antlers tags causing invalid XML

I have an XML template set up for an RSS feed. The generated XML is valid if I use static strings, but errors out when using Antlers tags. For instance:

This creates valid XML

# Template
<title>My Podcast</title>

# Browser Output
<?xml version="1.0" encoding="UTF-8"?>
<title>My Podcast</title>

This doesn't

# Template
<title>{{ site_title }}</title>

# Browser Output
&lt;?xml version="1.0" encoding="UTF-8"?&gt;
<title>My Podcast</title>

Neither does this

# Template
{{ collection:episodes limit="1" }}
  <title>Just a normal string</title>
{{ /collection:episodes }}

# Browser Output
&lt;?xml version="1.0" encoding="UTF-8"?&gt;
<title>Just a normal string</title>

See how the < gets escaped into <? Why is that? When I try to load the page, I get a nice red box with

XML Parsing Error: not well-formed
Location: http://localhost:1234/feed
Line Number 1, Column 1:

The document header has a Content Type of text/xml; charset=UTF-8, which looks correct to me. Here's how my route is set up:

/feed:
  layout: feed
  template: feed
  content_type: xml

Am I missing something about using Antlers in a document type other than HTML?