Source

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

protected function get_user( $id ) {
		$error = new WP_Error(
			'rest_user_invalid_id',
			__( 'Invalid user ID.' ),
			array( 'status' => 404 )
		);

		if ( (int) $id <= 0 ) {
			return $error;
		}

		$user = get_userdata( (int) $id );
		if ( empty( $user ) || ! $user->exists() ) {
			return $error;
		}

		if ( is_multisite() && ! is_user_member_of_blog( $user->ID ) ) {
			return $error;
		}

		return $user;
	}