Uncaught Error: Call to undefined function mb_strtoupper()
in /srv/www/dvojklik.cz/public/wp-content/plugins/wordpress-seo/src/generators/schema/article.php on line 225

Call stack:

1. Yoast\WP\SEO\Generators\Schema\Article::word_count()
   wp-content/plugins/wordpress-seo/src/generators/schema/article.php:60
2. Yoast\WP\SEO\Generators\Schema\Article::generate()
   wp-content/plugins/wordpress-seo/src/generators/schema-generator.php:125
3. Yoast\WP\SEO\Generators\Schema_Generator::generate_graph()
   wp-content/plugins/wordpress-seo/src/generators/schema-generator.php:72
4. Yoast\WP\SEO\Generators\Schema_Generator::generate()
   wp-content/plugins/wordpress-seo/src/presentations/indexable-presentation.php:720
5. Yoast\WP\SEO\Presentations\Indexable_Presentation::generate_schema()
   wp-content/plugins/wordpress-seo/src/presentations/abstract-presentation.php:66
6. Yoast\WP\SEO\Presentations\Abstract_Presentation::__get()
   wp-content/plugins/wordpress-seo/src/presenters/schema-presenter.php:61
7. Yoast\WP\SEO\Presenters\Schema_Presenter::get()
   wp-content/plugins/wordpress-seo/src/presenters/schema-presenter.php:45
8. Yoast\WP\SEO\Presenters\Schema_Presenter::present()
   wp-content/plugins/wordpress-seo/src/integrations/front-end-integration.php:486
9. Yoast\WP\SEO\Integrations\Front_End_Integration::present_head()
   wp-includes/class-wp-hook.php:353
10. WP_Hook::apply_filters()
    wp-includes/class-wp-hook.php:377
11. WP_Hook::do_action()
    wp-includes/plugin.php:523
12. do_action()
    wp-content/plugins/wordpress-seo/src/integrations/front-end-integration.php:457
13. Yoast\WP\SEO\Integrations\Front_End_Integration::call_wpseo_head()
    wp-includes/class-wp-hook.php:353
14. WP_Hook::apply_filters()
    wp-includes/class-wp-hook.php:377
15. WP_Hook::do_action()
    wp-includes/plugin.php:523
16. do_action()
    wp-includes/general-template.php:3441
17. wp_head()
    wp-content/themes/expertly/header.php:18
18. require_once('/srv/www/dvojklik.cz.../expertly/header.php')
    wp-includes/template.php:880
19. load_template()
    wp-includes/template.php:815
20. locate_template()
    wp-includes/general-template.php:48
21. get_header()
    wp-content/themes/expertly/single.php:8
22. include('/srv/www/dvojklik.cz.../expertly/single.php')
    wp-includes/template-loader.php:132
23. require_once('/srv/www/dvojklik.cz.../template-loader.php')
    wp-blog-header.php:19
24. require('/srv/www/dvojklik.cz/public/wp-blog-header.php')
    index.php:17