why does this CoffeeScript not accurately translate this Ruby?

wtf.coffee

insert: (ball, column) ->
  for row in @rows when row[column] is null
    row[column] = ball
    return

wtf.rb

def insert(ball, column)
  @grid.each do |row|
    next unless row[column].nil?
    row[column] = ball
    return
  end 
end 

wtf_spec.coffee

  it 'avoids "collisions" on insert', ->
    @grid = new Grid
    @ball = new Ball 1
    @ball2 = new Ball 2

    @grid.insert @ball, 0
    @grid.insert @ball2, 0

    expect(@grid.contents(0, 0)).toEqual(@ball)
    expect(@grid.contents(0, 1)).toEqual(@ball2)
    expect(@grid.contents(0, 2)).toEqual(null)