Undefined property: stdClass::$labels in general-template.php post_type_archive_title()

I’ve got a custom post type with a standard tag taxonomy added to it like so: 'taxonomies' => array('post_tag'). I’ve added some tags to some posts of this CPT which are display on the front-end with the template tag the_tags() and the links it generates have this format http://local.mysite.dev/tag/tag1/. When I click such a link a page without any posts is generated using the tag.php template, but if I add ?post_type=seron_mycpt to the end of the URL like so http://local.mysite.dev/tag/tag1/?post_type=seron_mycpt a page with the relevant posts is generated using the same template but this time I also get these lines added to debug.log: