diff --git a/defines.php b/defines.php
index a0cc1409..e2b4b939 100644
--- a/defines.php
+++ b/defines.php
@@ -12,7 +12,7 @@
 defined('ABSPATH') or die('No direct script access allowed.');
 
 if (!defined('PP_AUTHORS_VERSION')) {
-    define('PP_AUTHORS_VERSION', '3.6.2');
+    define('PP_AUTHORS_VERSION', '3.6.3');
     define('PP_AUTHORS_FILE', 'publishpress-authors/publishpress-authors.php');
     define('PP_AUTHORS_BASE_PATH', plugin_dir_path(__DIR__ . '/publishpress-authors.php'));
     define('PP_AUTHORS_MODULES_PATH', PP_AUTHORS_BASE_PATH . 'src/modules/');
diff --git a/publishpress-authors.php b/publishpress-authors.php
index d2dc677d..750c9437 100644
--- a/publishpress-authors.php
+++ b/publishpress-authors.php
@@ -5,7 +5,7 @@
  * Description: PublishPress Authors allows you to add multiple authors and guest authors to WordPress posts
  * Author:      PublishPress
  * Author URI:  https://publishpress.com
- * Version: 3.6.2
+ * Version: 3.6.3
  * Text Domain: publishpress-authors
  *
  * ------------------------------------------------------------------------------
diff --git a/readme.txt b/readme.txt
index c3d29e1a..efd71066 100644
--- a/readme.txt
+++ b/readme.txt
@@ -7,7 +7,7 @@ Tags: multiple authors, authors, guest authors, author fields, author layouts
 Requires at least: 4.7
 Requires PHP: 5.6
 Tested up to: 5.5
-Stable tag: 3.6.2
+Stable tag: 3.6.3
 License: GPLv2 or later
 License URI: http://www.gnu.org/licenses/gpl-2.0.html
 
@@ -127,6 +127,10 @@ There are two ways to install the PublishPress Authors plugin:
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning v2.0.0](https://semver.org/spec/v2.0.0.html).
 
+= [3.6.3] - 2020-09-04 =
+
+* Fixed: Fix error "Uncaught Error: Call to a member function add_cap() on null", #223;
+
 = [3.6.2] - 2020-09-03 =
 
 * Fixed: Fix error "Call to a member function get_error_message() on boolean", a regression bug result of the recent updates, #221;
diff --git a/src/core/Classes/Installer.php b/src/core/Classes/Installer.php
index c6e2cc84..be93c66e 100644
--- a/src/core/Classes/Installer.php
+++ b/src/core/Classes/Installer.php
@@ -24,6 +24,7 @@
 namespace MultipleAuthors\Classes;
 
 use MultipleAuthors\Classes\Objects\Author;
+use WP_Role;
 
 class Installer
 {
@@ -215,15 +216,19 @@ protected static function flush_permalinks()
     private static function add_new_edit_post_authors_cap()
     {
         $cap = 'ppma_edit_post_authors';
-
-        $role = get_role('author');
-        $role->add_cap($cap);
-
-        $role = get_role('editor');
-        $role->add_cap($cap);
-
-        $role = get_role('contributor');
-        $role->add_cap($cap);
+        $roles = [
+            'author',
+            'editor',
+            'contributor',
+        ];
+
+        foreach ($roles as $roleNmae)
+        {
+            $role = get_role($roleNmae);
+            if ($role instanceof WP_Role) {
+                $role->add_cap($cap);
+            }
+        }
     }
 
     /**