Crash casting 'Demonic Strike' on lone enemy

Had cast ‘Demonic Strike’ on the last enemy left in the battle, and the game crashed with the following:

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

Push :: Execution Error - Variable Index [0,1] out of range [1,1] - -7.crits(100001,1)
at gml_Script_bc_CountClass
############################################################################################

stack frame is
gml_Script_bc_CountClass (line -1)
gml_Script_bc_SpellDamage
gml_Script_bc_CreatureCast
gml_Script_bc_Turn
gml_Object_obj_battlecontroller_Step_0

Thanks, found the problem!