Crash when using Sorcery Shapeshifter

Doesn’t happen every time, but has crashed twice on me now

Here’s the log:

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

Variable obj_artifact.<unknown variable>(100064, -2147483648) not set before reading it.
at gml_Script_bc_GetAlliesOf
############################################################################################
--------------------------------------------------------------------------------------------
stack frame is
gml_Script_bc_GetAlliesOf (line -1)
gml_Script_scr_CritHasPassive
gml_Script_scr_AnyAllyHasPassive
gml_Script_bc_EventManaLoss
gml_Script_bc_EventManaRecover
gml_Script_bc_EventManaLoss
gml_Script_bc_EventManaRecover
gml_Script_bc_EventManaLoss
gml_Script_bc_EventManaRecover
gml_Script_bc_EventManaLoss
gml_Script_bc_EventManaRecover
gml_Script_bc_EventManaLoss
gml_Script_bc_EventManaRecover
gml_Script_bc_EventManaLoss
gml_Script_bc_EventManaRecover
gml_Script_bc_EventManaLoss
gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
gml_Script_bc_EventManaLoss
gml_Script_bc_EventManaRecover
gml_Script_bc_EventManaLoss
gml_Script_bc_EventManaRecover
gml_Script_bc_EventManaLoss
gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
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gml_Script_bc_EventManaRecover
gml_Script_bc_EventManaLoss
gml_Script_bc_EventManaRecover
gml_Script_bc_EventManaLoss
gml_Script_bc_EventManaRecover
gml_Script_bc_CreatureCast
gml_Script_bc_Turn
gml_Object_obj_battlecontroller_Step_0