[RC4] Arena error message spam

[Robsoie]

Playing a skeleton necromancer in arena 60 waves, when the monsters start to spawn, every turn a bunch of similar error message.
I reported, though in case it didn't make it, here the relevant part in the log :

[LOG] #YELLOW#The gates open!
[LOG]
Loading tile npc/vermin_rodent_giant_white_mouse.png
Loading tile npc/vermin_rodent_giant_crystal_rat.png
Loading tile npc/vermin_rodent_giant_brown_mouse.png
Loading tile npc/vermin_rodent_giant_grey_mouse.png
Lua Error: /engine/utils.lua bad argument #3 to 'new' (number expected, got nil)
At [C]:-1
At [C]:-1 new
At /engine/utils.lua:1844 new
At /engine/interface/ActorAI.lua:79 runAI
At /engine/ai//simple.lua:206 runAI
At /engine/ai//talented.lua:64 doAI
At /mod/class/NPC.lua:60 act
At /engine/GameEnergyBased.lua:131 tickLevel
At /engine/GameEnergyBased.lua:62 tick
At /engine/GameTurnBased.lua:46 tick
At /mod/class/Game.lua:1041
---------------- Stack Dump ----------------
2: table // 93c5e98
1: table // 93c5e98
--------------- Stack Dump Finished ---------------

spammed multiple time by turns.