Uppercases slot defines.

code/game/objects/items/shields.dm

@@ -10,7 +10,7 @@
 	icon_state = "riot"
 	lefthand_file = 'icons/mob/inhands/equipment/shields_lefthand.dmi'
 	righthand_file = 'icons/mob/inhands/equipment/shields_righthand.dmi'
-	slot_flags = SLOT_BACK
+	slot_flags = ITEM_SLOT_BACK
 	force = 10
 	throwforce = 5
 	throw_speed = 2
@@ -138,7 +138,7 @@
 		throwforce = 5
 		throw_speed = 2
 		w_class = WEIGHT_CLASS_BULKY
-		slot_flags = SLOT_BACK
+		slot_flags = ITEM_SLOT_BACK
 		to_chat(user, "<span class='notice'>You extend \the [src].</span>")
 	else
 		force = 3