(nil can't be coerced into Integer) getting this error when using prawn gem

Question

 @@pdf = Prawn::Document.new
 summary='s2_summary'
 @@pdf.text summary.to_s, :align => :left
 @@pdf.text "Dsh Days Breakdown", :align => :center

when running the code " @@pdf.text "Dsh Days Breakdown", :align => :center" getting (nil can't be coerced into Integer) error