Paperclip not displaying the error message

Question

I am using rails 3.2.6. I have added this in my models file:

 has_one :avatar, :as => :imageable, :dependent => :destroy
 validates_presence_of :avatar, :message => "can't be empty"

But I am not getting any error message if the field is left blank.I am only getting Please review the problems below: at the top of my page. Can anyone suggest anything?

has_one :avatar, :as => :imageable, :dependent => :destroy
  has_many :attachments, :as => :attachable, :dependent => :destroy

  accepts_nested_attributes_for :avatar, :reject_if => :all_blank

Actually how to give error message in nested attributes?