diff --git a/base/gui/src/main/java/cc/carm/lib/easyplugin/gui/configuration/GUIItemConfiguration.java b/base/gui/src/main/java/cc/carm/lib/easyplugin/gui/configuration/GUIItemConfiguration.java
index 5cd6950..1e6f607 100644
--- a/base/gui/src/main/java/cc/carm/lib/easyplugin/gui/configuration/GUIItemConfiguration.java
+++ b/base/gui/src/main/java/cc/carm/lib/easyplugin/gui/configuration/GUIItemConfiguration.java
@@ -46,7 +46,7 @@ public class GUIItemConfiguration {
         this.slots.forEach(slot -> gui.setItem(slot, item));
     }
 
-    public Map<String, Object> serialize() {
+    public @NotNull Map<String, Object> serialize() {
         LinkedHashMap<String, Object> map = new LinkedHashMap<>();
 
         map.put("type", this.type.name());