Where is the syntax error in this Ruby snippet?

Pulling my hair out...

Prawn::Document.generate(@targetfile) do |pdf|

  pdf.bounding_box ([80, 510] , :width => 400) do

    pdf.text("hello")

  end

end

gives a syntax error, unexpected ',', expecting ')' for the "comma" just before :width => 400

I tried this with both Ruby 1.9.3 and 2.1 - both give the same error. The only other thing I changed was that I upgraded the prawn version from 1.0 to 2.0 - according to the manual using prawn like this should still be ok though.