Rigid spreadsheets: impossible to create a blank book

I am using https://github.com/zdavatz/spreadsheet version 0.8.3

I have code that iterates through a collection and creates a worksheet in the same workbook for each item. When the collection is empty, the code that runs looks like this:

io = StringIO.new
book = Spreadsheet::Workbook.new
book.write(io)

The last line goes up:

TypeError: can't convert nil into Integer
    spreadsheet/excel/writer/workbook.rb:636:in `pack'
    spreadsheet/excel/writer/workbook.rb:636:in `write_window1'
    spreadsheet/excel/writer/workbook.rb:419:in `write_from_scratch'
    spreadsheet/excel/writer/workbook.rb:644:in `write_workbook'
    spreadsheet/writer.rb:12:in `write'
    spreadsheet/workbook.rb:124:in `write'

Changing my code to this fixes:

io = StringIO.new
book = Spreadsheet::Workbook.new
book.create_worksheet if book.worksheets.empty?
book.write(io)

But I do not want to include a call create_worksheet. My questions:

a) Am I doing something wrong in my code?

b) is it a stone error or an expected failure?