How to set a Prawn::Table::Cell height?

Question

I am using prawn to generate pdfs and I am having problems when settnig the table height, I set the height but nothing happens. I am using like this:

mensagem_tabela = msg_nfe + "\n" + msg_numero + "\n" + msg_serie
tabela = Prawn::Table
tabela = [[Prawn::Table::Cell.new(:text => mensagem_tabela, :height => 5.cm)]]
@data = [[Prawn::Table::Cell.new(:text => msg, :font_size => 6), tabela]]
pdf.table @data, :width => 20.cm , :vertical_padding => 2, :column_widths => {0 => 15.5.cm}, :align => {1 => :center}

But when I see the result the height tat a set is not printed in the pdf. Anybody knows why?