Rails adding a row to a csv that only has headers and no data removes the headers

I am reading a CSV file from a CDN and adding data to it then writing it back to a CDN for this reason I can't use append file.

I am also reordering the file which is why I have to turn it back into a CSV::Table

This code is a slimed down version of what I have but the problem is still replicated.

class SomeForm::Form
  class << self

    def add
      data = CSV.parse(open(csv_file_url).read, headers: true)
      data << ['david', 'smith'] # this will be dynamic
      # data = data.sort_by { |row| [row['Firstname'], row['Lastname']] }
      write_csv_file(CSV::Table.new(data).to_csv)
    end


    def csv_file_url
      #somecode to get the file_url
    end

    def write_csv_file(data)
      #somecode to write the file to the cdn
    end
  end
end

CSV file

Firstname,Lastname

So if I run SomeForm::Form.add with the csv file as above it outputs (removing the headers)

,
David,smith

However if the CSV file has at least one record in eg

Firstname,Lastname
Steve,Cougan

and run the code it adds the new record as expected

Firstname,Lastname
Steve,Cougan
david,smith

I would like to get this to work with csv with data in and csv with just the headers in.

As a aside this is for a rails3 project, though the above is replicatable in rails 5 as well

Thanks in advance