diff --git a/TODO.md b/TODO.md
index 6b6c286..9c4c25e 100644
--- a/TODO.md
+++ b/TODO.md
@@ -13,7 +13,7 @@
 | MS-09 |   Done   | Refactor all event listeners to use the modern event system (Listener interface, @EventHandler, registerEvents).
 | MS-10 |   Done   | Replace use of org.bukkit.util.config.Configuration with the modern configuration API (getConfig(), saveConfig(), etc.).
 | MS-11 |   Done   | Update score table to use Bukkit entity types or enums instead of CraftBukkit class names.
-| MS-12 |   Ready  | If storing player scores, refactor to use UUID as the key instead of Player or String.
+| MS-12 |   Done   | If storing player scores, refactor to use UUID as the key instead of Player or String.
 | MS-13 |   Done   | Build the plugin with Gradle (./gradlew build) and test on a modern Paper server using the provided bash scripts.
 | MS-14 |   Done   | Address any bugs or incompatibilities found during testing on a modern server.
 | MS-15 |   Ready  | Update README.md and CONTRIBUTING.md with new build, usage, and development instructions. _(Depends on: bugfixes)_
diff --git a/src/main/java/com/majinnaibu/minecraft/plugins/mobscores/MobScoresPlugin.java b/src/main/java/com/majinnaibu/minecraft/plugins/mobscores/MobScoresPlugin.java
index 354e2a0..47bff12 100644
--- a/src/main/java/com/majinnaibu/minecraft/plugins/mobscores/MobScoresPlugin.java
+++ b/src/main/java/com/majinnaibu/minecraft/plugins/mobscores/MobScoresPlugin.java
@@ -20,6 +20,7 @@ package com.majinnaibu.minecraft.plugins.mobscores;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.Map;
+import java.util.UUID;
 import java.util.logging.Level;
 
 import net.kyori.adventure.text.Component;
@@ -37,7 +38,7 @@ import com.majinnaibu.minecraft.plugins.mobscores.listeners.PlayerConnectListene
 import com.majinnaibu.minecraft.plugins.scorekeeper.ScoreKeeperPlugin;
 
 public class MobScoresPlugin extends JavaPlugin {
-	private Map<Entity, Player> _claimedMobs = new HashMap<Entity, Player>();
+	private Map<UUID, UUID> _claimedMobs = new HashMap<UUID, UUID>();
 	private Map<EntityType, Integer> _scoreTable = new HashMap<EntityType, Integer>();
 	private ScoreKeeperPlugin _scoreKeeper = null;
 
@@ -113,16 +114,20 @@ public class MobScoresPlugin extends JavaPlugin {
 
 	public void claimMob(Entity entity, Player damager) {
 		if(entity instanceof Zombie){
-			_claimedMobs.put((Zombie)entity, damager);
+			_claimedMobs.put(entity.getUniqueId(), damager.getUniqueId());
 		}	
 	}
 
 	public void awardScore(Entity entity) {
-
-		if(_claimedMobs.containsKey(entity)){
+		UUID playerId = _claimedMobs.remove(entity.getUniqueId());
+		if(playerId != null){
 			EntityType type = entity.getType();
 			if (_scoreTable.containsKey(type)) {
-				Player player = _claimedMobs.get(entity);
+				Player player = getServer().getPlayer(playerId);
+				if (player == null) {
+					logWarning("Unable to award score because the credited player is offline.");
+					return;
+				}
 				int score = _scoreTable.get(type);
 				_scoreKeeper.addScore(player, score);
 			} else {