diff --git a/activitypub-event-extensions.php b/activitypub-event-extensions.php
index 7981a5a..65424df 100644
--- a/activitypub-event-extensions.php
+++ b/activitypub-event-extensions.php
@@ -9,7 +9,7 @@
  * Text Domain: activitypub-event-extensions
  * License:     AGPL-3.0-or-later
  *
- * ActivityPub tested up to: 2.4.0
+ * ActivityPub tested up to: 3.1.0
  *
  * @package activitypub-event-extensions
  * @license AGPL-3.0-or-later
@@ -23,6 +23,7 @@ define( 'ACTIVITYPUB_EVENT_EXTENSIONS_PLUGIN_BASENAME', plugin_basename( __FILE_
 define( 'ACTIVITYPUB_EVENT_EXTENSIONS_PLUGIN_FILE', plugin_dir_path( __FILE__ ) . '/' . basename( __FILE__ ) );
 define( 'ACTIVITYPUB_EVENT_EXTENSIONS_PLUGIN_URL', plugin_dir_url( __FILE__ ) );
 define( 'ACTIVITYPUB_EVENT_EXTENSIONS_PLUGIN_VERSION', current( get_file_data( __FILE__, array( 'Version' ), 'plugin' ) ) );
+define( 'ACTIVITYPUB_EVENT_EXTENSIONS_DOMAIN', 'activitypub-event-extensions' );
 
 // Include and register the autoloader class for automatic loading of plugin classes.
 require_once ACTIVITYPUB_EVENT_EXTENSIONS_PLUGIN_DIR . '/includes/class-autoloader.php';
diff --git a/includes/class-settings.php b/includes/class-settings.php
index f5e781a..bfc1e82 100644
--- a/includes/class-settings.php
+++ b/includes/class-settings.php
@@ -24,10 +24,10 @@ defined( 'ABSPATH' ) || exit; // @codeCoverageIgnore
  * @since 1.0.0
  */
 class Settings {
-	const STATIC = 'Activitypub_Event_Extensions\Admin\Settings_Page';
-
 	const SETTINGS_SLUG = 'activitypub-event-extensions';
 
+	const DEFAULT_EVENT_CATEGORY = 'MEETING';
+
 	/**
 	 * Register the settings for the ActivityPub Event Extensions plugin.
 	 */
@@ -36,10 +36,11 @@ class Settings {
 			'activitypub-event-extensions',
 			'activitypub_event_extensions_default_event_category',
 			array(
-				'type'         => 'string',
-				'description'  => \__( 'Define your own custom post template', 'activitypub' ),
-				'show_in_rest' => true,
-				'default'      => 'MEETING',
+				'type'              => 'string',
+				'description'       => \__( 'Define your own custom post template', 'activitypub' ),
+				'show_in_rest'      => true,
+				'default'           => self::DEFAULT_EVENT_CATEGORY,
+				'sanitize_callback' => array( self::class, 'sanitize_mapped_event_category' ),
 			)
 		);
 
@@ -50,16 +51,45 @@ class Settings {
 				'type'              => 'array',
 				'description'       => \__( 'Define your own custom post template', 'activitypub' ),
 				'default'           => array(),
-				'sanitize_callback' => function ( $event_category_mappings ) {
-					$allowed_mappings = Event::DEFAULT_EVENT_CATEGORIES;
-					foreach ( $event_category_mappings as $key => $value ) {
-						if ( ! in_array( $value, $allowed_mappings, true ) ) {
-							unset( $event_category_mappings[ $key ] );
-						}
-					}
-					return $event_category_mappings;
-				},
+				'sanitize_callback' => array( self::class, 'sanitize_event_category_mappings' ),
 			)
 		);
 	}
+
+	/**
+	 * Sanitize the target ActivityPub Event category.
+	 *
+	 * @param string $event_category The ActivityPUb event category.
+	 */
+	public static function sanitize_mapped_event_category( $event_category ) {
+		return self::is_allowed_event_category( $event_category ) ? $event_category : self::DEFAULT_EVENT_CATEGORY;
+	}
+
+	/**
+	 * Sanitization function for the event category mapping setting.
+	 *
+	 * Currently only the default event categories are allowed to be target of a mapping.
+	 *
+	 * @param array $event_category_mappings The settings value.
+	 */
+	public static function sanitize_event_category_mappings( $event_category_mappings ) {
+		foreach ( $event_category_mappings as $taxonomy_slug => $event_category ) {
+			if ( ! self::is_allowed_event_category( $event_category ) ) {
+				unset( $event_category_mappings[ $taxonomy_slug ] );
+			}
+		}
+		return $event_category_mappings;
+	}
+
+	/**
+	 * Checks if the given event category is allowed to be target of a mapping.
+	 *
+	 * @param string $event_category The event category to check.
+	 *
+	 * @return bool True if allowed, false otherwise.
+	 */
+	private static function is_allowed_event_category( $event_category ) {
+		$allowed_event_categories = Event::DEFAULT_EVENT_CATEGORIES;
+		return in_array( $event_category, $allowed_event_categories, true );
+	}
 }