New DOMDocument totally disrupts WP

I’m trying to use DOMDocument in a plugin. Here’s the problem line:

$wpa_doc = new DOMDocument();

This works fine outside of WP on the same server, but even in an otherwise-empty plugin with only this line, visiting the site returns a totally blank page. Putting this in a content filter function displays everything up to the content.

Is there some sort of problem with using DOMDocument in WP?