diff --git a/eco-api/src/main/java/com/willfp/eco/core/gui/slot/CustomSlot.java b/eco-api/src/main/java/com/willfp/eco/core/gui/slot/CustomSlot.java
index b05ac4a4..6a85aefe 100644
--- a/eco-api/src/main/java/com/willfp/eco/core/gui/slot/CustomSlot.java
+++ b/eco-api/src/main/java/com/willfp/eco/core/gui/slot/CustomSlot.java
@@ -31,7 +31,7 @@ public abstract class CustomSlot implements Slot {
     }
 
     @Override
-    public ItemStack getItemStack(@NotNull final Player player) {
+    public @NotNull ItemStack getItemStack(@NotNull final Player player) {
         if (delegate == null) {
             throw new IllegalStateException("Custom Slot was not initialized!");
         }
diff --git a/eco-api/src/main/java/com/willfp/eco/core/gui/slot/ReactiveSlot.java b/eco-api/src/main/java/com/willfp/eco/core/gui/slot/ReactiveSlot.java
index 1af5db4a..dafdf8ea 100644
--- a/eco-api/src/main/java/com/willfp/eco/core/gui/slot/ReactiveSlot.java
+++ b/eco-api/src/main/java/com/willfp/eco/core/gui/slot/ReactiveSlot.java
@@ -29,7 +29,7 @@ public abstract class ReactiveSlot implements Slot {
                                  @NotNull final Menu menu);
 
     @Override
-    public ItemStack getItemStack(@NotNull final Player player) {
+    public @NotNull ItemStack getItemStack(@NotNull final Player player) {
         return new ItemStack(Material.STONE);
     }
 
diff --git a/eco-api/src/main/java/com/willfp/eco/core/gui/slot/Slot.java b/eco-api/src/main/java/com/willfp/eco/core/gui/slot/Slot.java
index 787653e3..0a0c9256 100644
--- a/eco-api/src/main/java/com/willfp/eco/core/gui/slot/Slot.java
+++ b/eco-api/src/main/java/com/willfp/eco/core/gui/slot/Slot.java
@@ -30,6 +30,7 @@ public interface Slot extends GUIComponent {
      * @param player The player.
      * @return The ItemStack.
      */
+    @NotNull
     ItemStack getItemStack(@NotNull Player player);
 
     /**
@@ -60,6 +61,7 @@ public interface Slot extends GUIComponent {
      * @param menu   The menu.
      * @return The slot.
      */
+    @NotNull
     default Slot getActionableSlot(@NotNull final Player player,
                                    @NotNull final Menu menu) {
         return this;
@@ -125,7 +127,9 @@ public interface Slot extends GUIComponent {
      *
      * @param provider The provider.
      * @return The builder.
+     * @deprecated This method was written incorrectly, should have been a Player + Menu function.
      */
+    @Deprecated(since = "6.45.0", forRemoval = true)
     static SlotBuilder builder(@NotNull final Function<Player, ItemStack> provider) {
         return Eco.get().createSlotBuilder((player, menu) -> provider.apply(player));
     }
diff --git a/eco-core/core-backend/src/main/kotlin/com/willfp/eco/internal/gui/menu/RenderedInventory.kt b/eco-core/core-backend/src/main/kotlin/com/willfp/eco/internal/gui/menu/RenderedInventory.kt
index 412d8e8d..08c85801 100644
--- a/eco-core/core-backend/src/main/kotlin/com/willfp/eco/internal/gui/menu/RenderedInventory.kt
+++ b/eco-core/core-backend/src/main/kotlin/com/willfp/eco/internal/gui/menu/RenderedInventory.kt
@@ -31,9 +31,7 @@ class RenderedInventory(
     val state = mutableMapOf<String, Any?>()
 
     fun render() {
-        val previousCaptive = captiveItems.toMap()
-
-        captiveItems.clear()
+        val newCaptive = mutableMapOf<GUIPosition, ItemStack>()
 
         for (row in (1..menu.rows)) {
             for (column in (1..menu.columns)) {
@@ -48,11 +46,11 @@ class RenderedInventory(
 
                     if (slot.isCaptiveFromEmpty) {
                         if (!actualItem.isEmpty) {
-                            captiveItems[position] = actualItem
+                            newCaptive[position] = actualItem
                         }
                     } else {
                         if (actualItem != renderedItem && !EmptyTestableItem().matches(actualItem)) {
-                            captiveItems[position] = actualItem
+                            newCaptive[position] = actualItem
                         }
                     }
                 } else {
@@ -61,6 +59,10 @@ class RenderedInventory(
             }
         }
 
+        val previousCaptive = captiveItems.toMap()
+        captiveItems.clear()
+        captiveItems.putAll(newCaptive)
+
         menu.runOnRender(player)
 
         // Run second render if captive items changed