diff --git a/includes/class-indieauth-authorize.php b/includes/class-indieauth-authorize.php
index e58d260..2beeb29 100644
--- a/includes/class-indieauth-authorize.php
+++ b/includes/class-indieauth-authorize.php
@@ -84,26 +84,25 @@ public function get_indieauth_response( $response ) {
 	 * Attached to the rest_authentication_errors filter. Passes through existing
 	 * errors registered on the filter.
 	 *
-	 * @param WP_Error|null Current error, or null.
+	 * @param WP_Error|null|true Current error, null or true.
 	 *
-	 * @return WP_Error|null Error if one is set, otherwise null.
+	 * @return WP_Error|null|true Error if one is set, unchanged otherwise.
 	 */
 	public function rest_authentication_errors( $error = null ) {
 		if ( is_user_logged_in() ) {
 			// Another OAuth2 plugin successfully authenticated.
-			return null;
-		}
-
-		if ( ! empty( $error ) ) {
 			return $error;
 		}
+
 		if ( is_wp_error( $this->error ) ) {
 			return $this->error;
 		}
+
 		if ( is_oauth_error( $this->error ) ) {
 			return $this->error->to_wp_error();
 		}
-		return null;
+
+		return $error;
 	}
 
 	/**