PDF ERROR

hi everyone i am trying for a pdf report and when i do it i got the
below error.Plz help me
undefined method `header' for #<Prawn::Document:0xb595d098>

This is the code where i'm getting error

pdf.header pdf.margin_box.top_left do
  if FileTest.exists?("#{RAILS_ROOT}/public/uploads/image/
institute_logo.jpg")
    logo = "#{RAILS_ROOT}/public/uploads/image/institute_logo.jpg"
  else
    logo = "#{RAILS_ROOT}/public/images/application/
app_fedena_logo.jpg"
  end
  @institute_name=Configuration.get_config_value('InstitutionName');

@institute_address=Configuration.get_config_value('InstitutionAddress');
  pdf.image logo, :position=>:left, :height=>50, :width=>50
  pdf.font "Helvetica" do
    info = [[@institute_name],
      [@institute_address]]
    pdf.move_up(50)
    pdf.fill_color "97080e"
    pdf.table info, :width => 400,
      :align => {0 => :center},
      :position => :center,
      :border_color => "FFFFFF"
    pdf.move_down(20)
    pdf.stroke_horizontal_rule
  end
end
i don't know whats the problem
my GEM list