为什么我用inject重构的ruby不能工作?

我试着做一些重构来将一个each块转换成一个inject,但是没有成功,我也不明白为什么。

class String
  # Build the word profile for the given word. The word profile is an array of
  # 26 integers -- each integer is a count of the number of times each letter
  # appears in the word. 
  #
  def profile
    profile = Array.new(26) { 0 }
    self.downcase.split(//).each do |letter|
      # only process letters a-z
      profile[letter.ord - 'a'.ord] += 1 unless letter.ord > 'z'.ord
    end
    profile
  end
end

我的重构失败了:

class String
  # Build the word profile for the given word. The word profile is an array of
  # 26 integers -- each integer is a count of the number of times each letter
  # appears in the word. 
  #
  def profile
    self.downcase.split(//).inject(Array.new(26) {0}) do |profile, letter|
      # only process letters a-z
      profile[letter.ord - 'a'.ord] += 1 unless letter.ord > 'z'.ord 
    end
  end
end

`block in profile': undefined method `[]=' for 1:Fixnum (NoMethodError)

如果我理解正确的话,它与重构版本中profile对象上的array引用操作符不同,这意味着传递给inject的初始化器不起作用。这种理解正确吗?如果是,为什么不呢?

谢谢!