set_format don't work in existing workbook

[markd2lp]

I have this code:

book = Spreadsheet.open "#{Rails.root.to_s}/public/modelos/geral.xls"
sheet = book.worksheet 0

But when i try change some format, like border collumn, dont work.

format = Spreadsheet::Format.new(:border => true)
sheet.row(x).set_format(y, format)

I saw that font weight was change to bold, but it dont was in format, and dont create any border.

Spreadsheet: 0.6.8
Rails: 3.0.9
Ruby: 1.8.7