Head tags plugin causes a critical error when using api #35

Trying to access the posts endpoint while having the 'REST API - Head Tags' plugin enabled will yield a Wordpress error message.

Tried to access posts endpoint:
https://markhjorth.com/wp-json/wp/v2/posts

Error message:
"There has been a critical error on your website."

Plugin version:
Version 1.1.3

Wordpress version:
Version 5.4.1

Yoast version:
Version 14.1

Stack trace:

Fatal error: Uncaught Error: Class 'DOMDocument' not found in /var/www/markhjorth.com/wp-content/plugins/rest-api-head-tags/includes/class-frontity-headtags.php:155 Stack trace: #0 /var/www/markhjorth.com/wp-content/plugins/rest-api-head-tags/includes/class-frontity-headtags.php(133): Frontity_Headtags->parse('\n\t<!-- This sit...') #1 /var/www/markhjorth.com/wp-content/plugins/rest-api-head-tags/includes/class-frontity-headtags.php(100): Frontity_Headtags->compute_headtags(Array) #2 /var/www/markhjorth.com/wp-content/plugins/rest-api-head-tags/includes/hooks/class-frontity-headtags-post-type-hooks.php(99): Frontity_Headtags->get_headtags('post_614', Array) #3 /var/www/markhjorth.com/wp-includes/rest-api/endpoints/class-wp-rest-controller.php(458): Frontity_Headtags_Post_Type_Hooks->get_post_headtags(Array, 'head_tags', Object(WP_REST_Request), 'post') #4 /var/www/markhjorth.com/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php(1795): WP_REST_Controller->add_additional_fields_to_object(Array, Object(WP_RES in /var/www/markhjorth.com/wp-content/plugins/rest-api-head-tags/includes/class-frontity-headtags.php on line 155