[语法]在代码块中报错说undefined method `+'

class RubyLearning
  def test_each
    dirs = Dir.glob('*')
    dirs.each { |dir| @dirs += ( dir + '\n' )}
    puts @dirs
  end

end

RubyLearning.new.test_each

错误信息

NoMethodError: undefined method `+' for nil:NilClass

method test_each in ruby_learning.rb at line 4
method each in ruby_learning.rb at line 4
method test_each in ruby_learning.rb at line 4
at top level in ruby_learning.rb at line 10

Program exited with code #1 after 0.04 seconds.