diff --git a/eco-core/core-plugin/src/main/kotlin/com/willfp/ecoarmor/sets/ArmorSet.kt b/eco-core/core-plugin/src/main/kotlin/com/willfp/ecoarmor/sets/ArmorSet.kt
index 7c44ef0..b84d570 100644
--- a/eco-core/core-plugin/src/main/kotlin/com/willfp/ecoarmor/sets/ArmorSet.kt
+++ b/eco-core/core-plugin/src/main/kotlin/com/willfp/ecoarmor/sets/ArmorSet.kt
@@ -24,6 +24,7 @@ import com.willfp.libreforge.ViolationContext
 import com.willfp.libreforge.conditions.Conditions
 import com.willfp.libreforge.effects.Effects
 import com.willfp.libreforge.notNullMapOf
+import com.willfp.libreforge.notNullMutableMapOf
 import org.bukkit.Bukkit
 import org.bukkit.Sound
 import org.bukkit.inventory.ItemStack
@@ -44,16 +45,16 @@ class ArmorSet(
     val regularHolder: Holder
 
     /** Items in set. */
-    private val items = notNullMapOf<ArmorSlot, ItemStack>()
+    private val items = notNullMutableMapOf<ArmorSlot, ItemStack>()
 
     /** Holders in set. */
-    private val slotHolders = notNullMapOf<ArmorSlot, Holder>()
+    private val slotHolders = notNullMutableMapOf<ArmorSlot, Holder>()
 
     /** Items in advanced set. */
-    private val advancedItems = notNullMapOf<ArmorSlot, ItemStack>()
+    private val advancedItems = notNullMutableMapOf<ArmorSlot, ItemStack>()
 
     /** Holders in advanced set. */
-    private val advancedSlotHolders = notNullMapOf<ArmorSlot, Holder>()
+    private val advancedSlotHolders = notNullMutableMapOf<ArmorSlot, Holder>()
 
     /** Advancement shard item. */
     val advancementShardItem: ItemStack
diff --git a/eco-core/core-plugin/src/main/kotlin/com/willfp/ecoarmor/upgrades/Tier.kt b/eco-core/core-plugin/src/main/kotlin/com/willfp/ecoarmor/upgrades/Tier.kt
index c694cd7..b582949 100644
--- a/eco-core/core-plugin/src/main/kotlin/com/willfp/ecoarmor/upgrades/Tier.kt
+++ b/eco-core/core-plugin/src/main/kotlin/com/willfp/ecoarmor/upgrades/Tier.kt
@@ -11,6 +11,7 @@ import com.willfp.eco.util.StringUtils
 import com.willfp.ecoarmor.sets.ArmorSlot
 import com.willfp.ecoarmor.sets.ArmorUtils.getCrystalTier
 import com.willfp.libreforge.notNullMapOf
+import com.willfp.libreforge.notNullMutableMapOf
 import org.bukkit.inventory.ItemStack
 import org.bukkit.persistence.PersistentDataType
 import java.util.*
@@ -50,7 +51,7 @@ class Tier(
     /**
      * Item properties.
      */
-    val properties = notNullMapOf<ArmorSlot, TierProperties>()
+    val properties = notNullMutableMapOf<ArmorSlot, TierProperties>()
 
     /**
      * Create a new Tier.
diff --git a/gradle.properties b/gradle.properties
index 086c55a..5b32655 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -1,5 +1,5 @@
 #libreforge-updater
-#Tue Mar 28 21:40:35 BST 2023
+#Tue Mar 28 22:06:41 BST 2023
 kotlin.code.style=official
 libreforge-version=4.0.0
 version=8.0.0