Source

File: wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php

public function check_reassign( $value, $request, $param ) {
		if ( is_numeric( $value ) ) {
			return $value;
		}

		if ( empty( $value ) || false === $value || 'false' === $value ) {
			return false;
		}

		return new WP_Error(
			'rest_invalid_param',
			__( 'Invalid user parameter(s).' ),
			array( 'status' => 400 )
		);
	}