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