diff --git a/src/main/java/net/minestom/server/data/Data.java b/src/main/java/net/minestom/server/data/Data.java
index 330647a6c..815c88eaf 100644
--- a/src/main/java/net/minestom/server/data/Data.java
+++ b/src/main/java/net/minestom/server/data/Data.java
@@ -1,23 +1,12 @@
 package net.minestom.server.data;
 
-import net.minestom.server.MinecraftServer;
-
 import java.util.concurrent.ConcurrentHashMap;
 
 public class Data {
 
-    protected static final DataManager DATA_MANAGER = MinecraftServer.getDataManager();
-
     protected ConcurrentHashMap<String, Object> data = new ConcurrentHashMap();
 
     public <T> void set(String key, T value, Class<T> type) {
-        // Type registering is only relevant if data should be serialized
-        if (this instanceof SerializableData) {
-            if (DATA_MANAGER.getDataType(type) == null) {
-                throw new UnsupportedOperationException("Type " + type.getName() + " hasn't been registered in DataManager#registerType");
-            }
-        }
-
         this.data.put(key, value);
     }
 
diff --git a/src/main/java/net/minestom/server/data/SerializableData.java b/src/main/java/net/minestom/server/data/SerializableData.java
index 66c84db24..c3188e868 100644
--- a/src/main/java/net/minestom/server/data/SerializableData.java
+++ b/src/main/java/net/minestom/server/data/SerializableData.java
@@ -1,5 +1,6 @@
 package net.minestom.server.data;
 
+import net.minestom.server.MinecraftServer;
 import net.minestom.server.network.packet.PacketWriter;
 import net.minestom.server.utils.PrimitiveConversion;
 
@@ -11,10 +12,16 @@ import java.util.concurrent.ConcurrentHashMap;
 
 public class SerializableData extends Data {
 
+    private static final DataManager DATA_MANAGER = MinecraftServer.getDataManager();
+
     private ConcurrentHashMap<String, Class> dataType = new ConcurrentHashMap<>();
 
     @Override
     public <T> void set(String key, T value, Class<T> type) {
+        if (DATA_MANAGER.getDataType(type) == null) {
+            throw new UnsupportedOperationException("Type " + type.getName() + " hasn't been registered in DataManager#registerType");
+        }
+
         super.set(key, value, type);
         this.dataType.put(key, type);
     }
diff --git a/src/main/java/net/minestom/server/entity/Player.java b/src/main/java/net/minestom/server/entity/Player.java
index 06f7b6ab0..f35a8b9b4 100644
--- a/src/main/java/net/minestom/server/entity/Player.java
+++ b/src/main/java/net/minestom/server/entity/Player.java
@@ -799,7 +799,7 @@ public class Player extends LivingEntity {
         if (openInventory == null) {
             closeWindowPacket.windowId = 0;
         } else {
-            closeWindowPacket.windowId = (byte) openInventory.getInventoryType().getWindowType();
+            closeWindowPacket.windowId = openInventory.getWindowId();
             openInventory.removeViewer(this);
             refreshOpenInventory(null);
         }
diff --git a/src/main/java/net/minestom/server/inventory/Inventory.java b/src/main/java/net/minestom/server/inventory/Inventory.java
index 213b6d8c4..15e168235 100644
--- a/src/main/java/net/minestom/server/inventory/Inventory.java
+++ b/src/main/java/net/minestom/server/inventory/Inventory.java
@@ -20,7 +20,7 @@ public class Inventory implements InventoryModifier, InventoryClickHandler, View
 
     private static volatile byte lastInventoryId;
 
-    private int id;
+    private byte id;
     private InventoryType inventoryType;
     private String title;
 
@@ -66,7 +66,7 @@ public class Inventory implements InventoryModifier, InventoryClickHandler, View
         return title;
     }
 
-    public int getWindowId() {
+    public byte getWindowId() {
         return id;
     }
 
@@ -134,7 +134,7 @@ public class Inventory implements InventoryModifier, InventoryClickHandler, View
             itemStack = itemStack == null ? ItemStack.getAirItem() : itemStack;
             this.itemStacks[slot] = itemStack;
             SetSlotPacket setSlotPacket = new SetSlotPacket();
-            setSlotPacket.windowId = 1;
+            setSlotPacket.windowId = getWindowId();
             setSlotPacket.slot = (short) slot;
             setSlotPacket.itemStack = itemStack;
             sendPacketToViewers(setSlotPacket);