Ruby on Rails

consider scenario,

User model with name, birth_date fields (here birth_date is not mandatory field)

inside view form birth_date is assigned as '31/31/1985' which is invalid

ideally user object should be invalid and while save raise an error on birth_date field but that's not happening and user object gets saved with birth_date as blank which is completely misleading.

After debugging found that while assigning attributes birth_date value it gets assigned as blank and as birth_date is optional object gets saved.

class User < ActiveRecord::Base
    def initialize(args={})
    logger.info args[:birth_date] #invalid value comes upto here but vanishes afterwords
      super
  end
end

Any clue how to get validation working properly for not mandatory date fields ??