darkness item experience bug

If we give several darkness item to the Mr.Iron to construct, no matter how many the experience rate is, Mr.Iron will get negative experience. So, after kill creeps, the Mr.Iron will level down, even if I sold the dark tower..:( Is it right? Level 74 wave 327 All the creeps are killed by Mr.Iron wave 328 But he get level dwon. :( level 73