diff --git a/shop.lua b/shop.lua
index 881669f..af2338f 100644
--- a/shop.lua
+++ b/shop.lua
@@ -278,6 +278,7 @@ core.register_on_player_receive_fields(function(sender, formname, fields)
 	 -- set or reset timer
 	 core.get_node_timer(pos):start(shop_timer)
 
+	 -- set item count
 	 if fields.count and string.find(fields.count, "^[0-9]+$") then
 	    if new_count >= 1 and new_count <= 64 and new_count ~= meta:get_int("count") then
 	       meta:set_int("count", new_count)
@@ -285,31 +286,40 @@ core.register_on_player_receive_fields(function(sender, formname, fields)
 	    end
 	 end
 
+	 -- set price
 	 if fields.price and string.find(fields.price, "^[0-9]+$") then
 	    if new_price >= 1 and new_price <= 10000 and new_price ~= meta:get_int("price") then
 	       meta:set_int("price", new_price)
 	    end
 	 end
-	 
+
+	 -- someone hit exchange button
 	 if fields.exchange ~= nil and fields.exchange ~= "" then
 
+	    -- owner try exchanges
 	    if meta:get_string("owner") == name then
-
 	       core.chat_send_player(name, S("This is your own shop, you can't exchange to yourself!"))
 
 	    else
 	       
 	       local can_exchange = true
 
+	       -- have u money?
 	       if bankemeralds < old_price then
 		  can_exchange = false
 		  core.chat_send_player(name, S("Not enough Emeralds in your account") )
 	       end
-	       
+
+	       --there are enough items?
 	       if not minv:contains_item("stock", shop_item.." "..old_count, true) then
 		  can_exchange = false
 		  core.chat_send_player(name, S("Out of Stock!") )
 	       end
+
+	       -- do not trade air
+	       if shop_item == "" or shop_item == "air" then
+		  can_exchange = false
+	       end
 	       
 	       if can_exchange then
 		  minv:remove_item("stock", shop_item.." "..old_count)