Error: Cannot retrieve value of undefined directive HTML.Forms

I'm seeing a lot of this error in my logs

User warning: Cannot retrieve value of undefined directive HTML.Forms invoked on line 31 in file
/var/www/example.org/sites/all/civicrm_extensions/advimport/vendor/ezyang/htmlpurifier/library/HTMLPurifier/HTMLModule/Forms.php
in HTMLPurifier_Config->triggerError() (line 901 of
/var/www/example.org/sites/all/civicrm_extensions/advimport/vendor/ezyang/htmlpurifier/library/HTMLPurifier/Config.php)

I'm using the master branch, and have used composer install to add the vendor/ files. I also tried composer upgrade to no avail (htmlpurifier still at v4.13.0).