diff --git a/mods/ITEMS/mcl_stairs/api.lua b/mods/ITEMS/mcl_stairs/api.lua
index bebe7ba99..390461c6c 100644
--- a/mods/ITEMS/mcl_stairs/api.lua
+++ b/mods/ITEMS/mcl_stairs/api.lua
@@ -237,30 +237,43 @@ function mcl_stairs.register_slab(subname, recipeitem, groups, images, descripti
 			fixed = {-0.5, -0.5, -0.5, 0.5, 0, 0.5},
 		},
 		on_place = function(itemstack, placer, pointed_thing)
-			local under = minetest.get_node(pointed_thing.under)
+			if not placer then return end
+
+			local above = pointed_thing.above
+			local under = pointed_thing.under
+			local anode = minetest.get_node(above)
+			local unode = minetest.get_node(under)
+			local adefs = minetest.registered_nodes[anode.name]
+			local udefs = minetest.registered_nodes[unode.name]
 			local wield_item = itemstack:get_name()
-			local creative_enabled = minetest.is_creative_enabled(placer:get_player_name())
+			local player_name = placer:get_player_name()
+			local creative_enabled = minetest.is_creative_enabled(player_name)
 
 			-- place slab using under node orientation
-			local dir = vector.subtract(pointed_thing.above, pointed_thing.under)
+			local dir = vector.subtract(above, under)
+			local p2 = unode.param2
 
-			local p2 = under.param2
+			if minetest.is_protected(under, player_name) and not
+				minetest.check_player_privs(placer, "protection_bypass") then
+					minetest.record_protection_violation(under, player_name)
+					return
+			end
 
 			-- combine two slabs if possible
 			-- Requirements: Same slab material, must be placed on top of lower slab, or on bottom of upper slab
-			if (wield_item == under.name or (minetest.registered_nodes[under.name] and wield_item == minetest.registered_nodes[under.name]._mcl_other_slab_half)) and
-					not ((dir.y >= 0 and minetest.get_item_group(under.name, "slab_top") == 1) or
-					(dir.y <= 0 and minetest.get_item_group(under.name, "slab_top") == 0)) then
+			if (wield_item == unode.name or (udefs and wield_item == udefs._mcl_other_slab_half)) and
+					not ((dir.y >= 0 and minetest.get_item_group(unode.name, "slab_top") == 1) or
+					(dir.y <= 0 and minetest.get_item_group(unode.name, "slab_top") == 0)) then
 
-				local player_name = placer:get_player_name()
-				if minetest.is_protected(pointed_thing.under, player_name) and not
-						minetest.check_player_privs(placer, "protection_bypass") then
-					minetest.record_protection_violation(pointed_thing.under,
-						player_name)
-					return
+				minetest.set_node(under, {name = double_slab, param2 = p2})
+
+				if not creative_enabled then
+					itemstack:take_item()
 				end
-				local newnode = double_slab
-				minetest.set_node(pointed_thing.under, {name = newnode, param2 = p2})
+				return itemstack
+			elseif (wield_item == anode.name or (adefs and wield_item == adefs._mcl_other_slab_half)) then
+				minetest.set_node(above, {name = double_slab, param2 = p2})
+
 				if not creative_enabled then
 					itemstack:take_item()
 				end