diff --git a/README.md b/README.md
index 26f345d..6d0e87d 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # i3
 
-[![ContentDB](https://content.minetest.net/packages/jp/i3/shields/title/)](https://content.minetest.net/packages/jp/i3/) [![ContentDB](https://content.minetest.net/packages/jp/i3/shields/downloads/)](https://content.minetest.net/packages/jp/i3/) ![workflow](https://github.com/minetest-mods/i3/actions/workflows/luacheck.yml/badge.svg)
+[![ContentDB](https://content.minetest.net/packages/jp/i3/shields/downloads/)](https://content.minetest.net/packages/jp/i3/) ![workflow](https://github.com/minetest-mods/i3/actions/workflows/luacheck.yml/badge.svg)
 
 #### **`i3`** is a next-generation inventory for Minetest.
 
diff --git a/src/api.lua b/src/api.lua
index 47f242b..8e84077 100644
--- a/src/api.lua
+++ b/src/api.lua
@@ -302,6 +302,8 @@ function i3.add_sorting_method(def)
 		return err "i3.add_sorting_method: name missing"
 	elseif not is_func(def.func) then
 		return err "i3.add_sorting_method: function missing"
+	elseif i3.compressed[def.name] then
+		return err(fmt("i3.add_sorting_method: item '%s' is already compressed", def.name))
 	end
 
 	insert(i3.sorting_methods, def)