`require 'csv'; CSV.generate { |csv| csv << {a: 1, b: 2} }` throws `undefined method 'collect' for nil`

What's weird is that it works with structs (and arrays of course) but not with hashes ``` >> require 'csv'; CSV.generate { |csv| csv << {a: 1, b: 2} } /Users/dorianmariefr/.asdf/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/csv-3.3.0/lib/csv/writer.rb:36:in `<<': undefined method `collect' for nil (NoMethodError) row = @headers.collect {|header| row[header]} ^^^^^^^^ from /Users/dorianmariefr/.asdf/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/csv-3.3.0/lib/csv.rb:2373:in `<<' from (irb):2:in `block in <top (required)>' from /Users/dorianmariefr/.asdf/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/csv-3.3.0/lib/csv.rb:1410:in `generate' from (irb):2:in `<main>' from <internal:kernel>:187:in `loop' from /Users/dorianmariefr/.asdf/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/irb-1.14.0/exe/irb:9:in `<top (required)>' from /Users/dorianmariefr/.asdf/installs/ruby/3.3.4/bin/irb:25:in `load' from /Users/dorianmariefr/.asdf/installs/ruby/3.3.4/bin/irb:25:in `<main>' ```