diff --git a/README.md b/README.md
index 00c9fa8..977a57c 100644
--- a/README.md
+++ b/README.md
@@ -4,6 +4,11 @@ Don't you just hate it when your beautiful HTML markup is mutilated due to a tem
 
 Use this plugin to indent the HTML markup output by WordPress using the [Dindent](https://github.com/gajus/dindent) library. It only does its thing when you are not logged in.
 
+## Requirements
+
+* PHP 5.3 or greater
+* Mbstring extension
+
 ## Installation
 
 ### Composer (with [Bedrock](https://roots.io/bedrock/))
diff --git a/html-markup-indenter.php b/html-markup-indenter.php
index f949eb6..6dea16a 100644
--- a/html-markup-indenter.php
+++ b/html-markup-indenter.php
@@ -3,7 +3,7 @@
 Plugin Name: HTML Markup Indenter
 Plugin URI: https://github.com/joppuyo/html-markup-cleaner
 Description: Indents the HTML markup output by WordPress
-Version: 1.0.1
+Version: 2.0.0
 Author: Johannes Siipola
 Author URI: https://siipo.la
 */
@@ -20,7 +20,7 @@
 });
 
 add_action('shutdown', function () {
-    if (html_markup_indenter_is_html() && !is_user_logged_in()) {
+    if (extension_loaded('mbstring') && html_markup_indenter_is_html() && !is_user_logged_in()) {
         $final = '';
         $levels = ob_get_level();
         for ($i = 0; $i < $levels; $i++) {
@@ -39,3 +39,11 @@ function html_markup_indenter_is_html() {
     }
     return false;
 }
+
+add_action('admin_notices', function() {
+    if (!extension_loaded('mbstring')) {
+        echo '<div class="notice notice-warning"><p>';
+        echo __("Mbstring PHP extension is not loaded. HTML Markup Indenter has been disabled.", 'html-markup-indenter');
+        echo '</p></div>';
+    }
+});
diff --git a/readme.txt b/readme.txt
index d1e1463..c39d42e 100644
--- a/readme.txt
+++ b/readme.txt
@@ -6,14 +6,22 @@ Tested up to: 4.9.4
 Requires PHP: 5.3.0 or greater
 License: License: GPLv3 or later
 
-Indents the HTML markup output by WordPress
+Indents the HTML markup output by WordPress.
 
 == Description ==
-Indents the HTML markup output by WordPress using the [Dindent](https://github.com/gajus/dindent) library. It only does its thing when you are not logged in.
+Indents the HTML markup output by WordPress using the [Dindent](https://github.com/gajus/dindent) library. It only does its thing when you are not logged in. Requires PHP mbstring extension to be installed.
 
 == Installation ==
 1. Upload the plugin folder to the /wp-content/plugins/ directory
 2. Activate the plugin through the Plugins menu in WordPress
 
+== Changelog ==
+
+= 2.0.0 =
+* Make Mbstring extension a requirement
+
+= 1.0.1 =
+* Only indent HTML markup
+
 = 1.0.0 =
 * Initial release