[VX] method error in Tankentai atb 1.2c

Hello you wizards out there,
During my latest test battle (in-game) against a new enemy, I usually get a method error around halfway of the battle. It refers to the ATB 1.2c script for Tankentai SBS. The error is about the following line:

Spoiler for:

  @base_act_count = base *  100 / act_type[1]

The whole section the line is part from:

Spoiler for:

def change_act(act_type)
    change_atb
    case act_type[0]
    when 0
      base = @battler.at_speed * @battler.agi
      base = @battler.at_speed * @battler.agi / act_type[2] if act_type[2] != 0
    when 1
      base = @battler.at_speed * @battler.agi / @battler.speed_level
    when 2
      base = @battler.at_speed * @battler.agi if @battler.low_speed_level == 0
      base = @battler.at_speed * @battler.agi / @battler.low_speed_level
    when 3
      base = @battler.at_speed * @battler.agi if @battler.speed_level / 2 + @battler.low_speed_level / 2 == 0
      base = @battler.at_speed * @battler.agi / (@battler.speed_level / 2 + @battler.low_speed_level / 2)
    when 4
      base = @battler.at_speed
    end
    if act_type[1] <= 0
      @base_act_count = 1000
    else
      @base_act_count = base *  100 / act_type[1]
    end
    @base_act_count = 1 if @base_act_count == 0
    @battler.act_count = 0
    @gauge.bitmap = Cache.system("act_bar")
    @battler.atb_count_up = false
    @battler.at_active = false
  end

This is the first time it happens. The only new addition to this battle (apart from new enemy skills etc) is that one temporary teammate is added that auto-battles.

Can anyone help me out why this error persists, and explain to me how I can get rid of it?

Much obliged,
~ Geoff

Problem update: Seems it had to do with one of the skills the aut-character used. Removing it seems to have done the trick.

I do wonder about that though. The skill would give the whole tea + defense; why would that not work?