class Prawn::Table::Cell

pdf.table([["foo", "bar"], ["baz"]]) { cells[0, 1].rowspan = 2 }

has already run. For example, this will NOT work:
Setting rowspan from the initializer block is invalid because layout

pdf.table([["foo", {:content => "bar", :rowspan => 2}], ["baz"]])

This must be provided as part of the table data, like so:

Indicates the number of rows that this cell is to span. Defaults to 1.

def rowspan=(span)
  if defined?(@initializer_run) && @initializer_run
    raise Prawn::Errors::InvalidTableSpan,
      "rowspan must be provided in the table's structure, never in the " +
      "initialization block. See Prawn's documentation for details."
  end
  @rowspan = span
end