Uppercases slot defines.

code/game/objects/items/chrono_eraser.dm

@@ -9,7 +9,7 @@
 	lefthand_file = 'icons/mob/inhands/equipment/backpack_lefthand.dmi'
 	righthand_file = 'icons/mob/inhands/equipment/backpack_righthand.dmi'
 	w_class = WEIGHT_CLASS_BULKY
-	slot_flags = SLOT_BACK
+	slot_flags = ITEM_SLOT_BACK
 	slowdown = 1
 	actions_types = list(/datum/action/item_action/equip_unequip_TED_Gun)
 	var/obj/item/gun/energy/chrono_gun/PA = null
@@ -38,7 +38,7 @@
 				user.put_in_hands(PA)
 
 /obj/item/chrono_eraser/item_action_slot_check(slot, mob/user)
-	if(slot == slot_back)
+	if(slot == SLOT_BACK)
 		return 1
 
 /obj/item/gun/energy/chrono_gun