I'm refactoring some code and this proc is causing an error randomly and I don't know why or how to debug it... Any ideas?

New code with proc

  defense_moves, offense_moves = [], []
  determine_move = ->move,side,i { side << move.count(move[i]) }
  defense.size.times { |i| determine_move.(defense, defense_moves, i) }
  offense.size.times { |i| determine_move.(offense, offense_moves, i) }
  dm = defense[defense_moves.index(defense_moves.max)].nil? ? [0] : defense[defense_moves.index(defense_moves.max)]
  om = offense[offense_moves.index(offense_moves.max)].nil? ? [0] : offense[offense_moves.index(offense_moves.max)]

Original code:

  d = 0
  defense_moves = []
  loop do 
    defense_moves << defense.count(defense[d])
    break if defense.count(defense[d]).zero?
    d += 1
  end

  o = 0
  offense_moves = []
  loop do 
    offense_moves << offense.count(offense[o])
    break if offense.count(offense[o]).zero?
    o += 1
  end

  dm = defense[defense_moves.index(defense_moves.max)].nil? ? [0] : defense[defense_moves.index(defense_moves.max)]
  om = offense[offense_moves.index(offense_moves.max)].nil? ? [0] : offense[offense_moves.index(offense_moves.max)]

TypeError

ttt2.rb:95:in `[]': no implicit conversion from nil to integer (TypeError)
    from ttt2.rb:95:in `computer_make_move'
    from ttt2.rb:133:in `draw_board'
    from ttt2.rb:24:in `place'
    from ttt2.rb:209:in `block in start_new_game'
    from ttt2.rb:188:in `loop'
    from ttt2.rb:188:in `start_new_game'
    from ttt2.rb:199:in `block in start_new_game'
    from ttt2.rb:188:in `loop'
    from ttt2.rb:188:in `start_new_game'
    from ttt2.rb:199:in `block in start_new_game'
    from ttt2.rb:188:in `loop'
    from ttt2.rb:188:in `start_new_game'
    from ttt2.rb:199:in `block in start_new_game'
    from ttt2.rb:188:in `loop'
    from ttt2.rb:188:in `start_new_game'
    from ttt2.rb:199:in `block in start_new_game'
    from ttt2.rb:188:in `loop'
    from ttt2.rb:188:in `start_new_game'
    from ttt2.rb:234:in `<main>'