Crash in itherian realm

Trying to cast Etheral Knives against Smoke Devil boss. It was a no-mana-regen sigil, but my mana had regenerated for the boss fight


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FATAL ERROR in
action number 1
of Step Event0
for object obj_battlecontroller:

Variable obj_battlecontroller.source(100070, -2147483648) not set before reading it.
at gml_Script_bc_EventDamage
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stack frame is
gml_Script_bc_EventDamage (line -1)
gml_Script_bc_CreatureCast
gml_Script_bc_Turn
gml_Object_obj_battlecontroller_Step_0