diff --git a/includes/class-webfinger.php b/includes/class-webfinger.php
index b6b0a64..75f7ff6 100644
--- a/includes/class-webfinger.php
+++ b/includes/class-webfinger.php
@@ -41,9 +41,14 @@ class Webfinger {
 	 * @return string|WP_Error The URL or WP_Error
 	 */
 	public static function resolve( $resource ) {
+		if ( ! $resource ) {
+			return null;
+		}
+
 		if ( ! preg_match( '/^@?' . ACTIVITYPUB_USERNAME_REGEXP . '$/i', $resource, $m ) ) {
 			return null;
 		}
+
 		$transient_key = 'activitypub_resolve_' . ltrim( $resource, '@' );
 
 		$link = \get_transient( $transient_key );
diff --git a/includes/rest/class-inbox.php b/includes/rest/class-inbox.php
index 2cedf15..15fb4b2 100644
--- a/includes/rest/class-inbox.php
+++ b/includes/rest/class-inbox.php
@@ -236,8 +236,12 @@ class Inbox {
 		$params['actor'] = array(
 			'required' => true,
 			'sanitize_callback' => function( $param, $request, $key ) {
-				if ( ! \is_string( $param ) ) {
-					$param = $param['id'];
+				if ( \is_array( $param ) ) {
+					if ( isset( $param['id'] ) ) {
+						$param = $param['id'];
+					} else {
+						$param = $param['url'];
+					}
 				}
 				return \esc_url_raw( $param );
 			},
diff --git a/integration/class-webfinger.php b/integration/class-webfinger.php
index 177b417..c9dd565 100644
--- a/integration/class-webfinger.php
+++ b/integration/class-webfinger.php
@@ -30,6 +30,10 @@ class Webfinger {
 	public static function add_user_discovery( $array, $resource, $user ) {
 		$user = User_Collection::get_by_id( $user->ID );
 
+		if ( ! $user || is_wp_error( $user ) ) {
+			return $array;
+		}
+
 		$array['links'][] = array(
 			'rel'  => 'self',
 			'type' => 'application/activity+json',