diff --git a/includes/class-user-factory.php b/includes/class-user-factory.php
index ce70ccb..4a0ce5a 100644
--- a/includes/class-user-factory.php
+++ b/includes/class-user-factory.php
@@ -30,24 +30,32 @@ class User_Factory {
 	 * @return \Acitvitypub\Model\User The User.
 	 */
 	public static function get_by_id( $user_id ) {
-		$user_id = (int) $user_id;
-
-		if ( self::BLOG_USER_ID === $user_id ) {
-			return new Blog_User( $user_id );
-		} elseif ( self::APPLICATION_USER_ID === $user_id ) {
-			return new Application_User( $user_id );
-		} else {
-			$user = get_user_by( 'ID', $user_id );
-			if ( ! $user || ! \user_can( $user, 'publish_posts' ) ) {
-				return new WP_Error(
-					'activitypub_user_not_found',
-					\__( 'User not found', 'activitypub' ),
-					array( 'status' => 404 )
-				);
-			}
-
-			return new User( $user->ID );
+		if ( is_string( $user_id ) || is_numeric( $user_id ) ) {
+			$user_id = (int) $user_id;
 		}
+
+		if (
+			self::BLOG_USER_ID === $user_id &&
+			is_user_enabled( $user_id )
+		) {
+			return new Blog_User( $user_id );
+		} elseif (
+			self::APPLICATION_USER_ID === $user_id &&
+			is_user_enabled( $user_id )
+		) {
+			return new Application_User( $user_id );
+		} elseif (
+			$user_id > 0 &&
+			is_user_enabled( $user_id )
+		) {
+			return new User( $user_id );
+		}
+
+		return new WP_Error(
+			'activitypub_user_not_found',
+			\__( 'User not found', 'activitypub' ),
+			array( 'status' => 404 )
+		);
 	}
 
 	/**
diff --git a/includes/functions.php b/includes/functions.php
index 595e85b..7fc62d8 100644
--- a/includes/functions.php
+++ b/includes/functions.php
@@ -277,3 +277,49 @@ function is_activitypub_request() {
 function is_single_user_mode() {
 	return ACTIVITYPUB_SINGLE_USER_MODE;
 }
+
+/**
+ * This function checks if a user is enabled for ActivityPub.
+ *
+ * @param int $user_id The User-ID.
+ *
+ * @return boolean True if the user is enabled, false otherwise.
+ */
+function is_user_enabled( $user_id ) {
+	switch ( $user_id ) {
+		// if the user is the application user, it's always enabled.
+		case \Activitypub\User_Factory::APPLICATION_USER_ID:
+			return true;
+		// if the user is the blog user, it's only enabled in single-user mode.
+		case \Activitypub\User_Factory::BLOG_USER_ID:
+			if ( is_single_user_mode() ) {
+				return true;
+			}
+
+			return false;
+		// if the user is any other user, it's enabled if it can publish posts.
+		default:
+			if (
+				! is_single_user_mode() &&
+				\user_can( $user_id, 'publish_posts' )
+			) {
+				return true;
+			}
+
+			return false;
+	}
+}
+
+if ( ! function_exists( 'get_self_link' ) ) {
+	/**
+	 * Get the correct self URL
+	 *
+	 * @return boolean
+	 */
+	function get_self_link() {
+		$host = wp_parse_url( home_url() );
+
+		return esc_url( apply_filters( 'self_link', set_url_scheme( 'http://' . $host['host'] . wp_unslash( $_SERVER['REQUEST_URI'] ) ) ) );
+	}
+}
+
diff --git a/includes/model/class-post.php b/includes/model/class-post.php
index 5ebeb34..5004162 100644
--- a/includes/model/class-post.php
+++ b/includes/model/class-post.php
@@ -197,7 +197,7 @@ class Post {
 	 * @return int the User ID.
 	 */
 	public function get_user_id() {
-		return apply_filters( 'activitypub_post_user_id', $this->post_author, $this->post );
+		return apply_filters( 'activitypub_post_user_id', $this->get_post_author(), $this->post );
 	}
 
 	/**