Here's my guess as to what was wrong:
I was trying to use Pet on the Sunstone Guardian (that spawns a chest) and there were 5 enemies on the table (so, no room.)
Version 2.0.29
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ERROR in
action number 1
of Mouse Event for Left Pressed
for object obj_dieTargetArea:
Variable Get 100695 (101163, -2147483648)############################################################################################
gml_Script_purge_dead_enemies (line 388)
gml_Script_execute_ability (line 1275)
gml_Object_obj_dieTargetArea_Mouse_4 (line 6)