Incorrect return value of update_instance()

  /**
     * Updates a new assignment activity
     *
     * Given an object containing all the necessary data,
     * (defined by the form in mod.html) this function
     * will update the assignment instance and return the id number
     * The due date is updated in the calendar
     * This is common to all assignment types.
     *
     * @param $assignment object The data from the form on mod.html
     * @return int The assignment id
     */