diff --git a/runelite-client/src/main/java/net/runelite/client/plugins/chatnotifications/ChatNotificationsPlugin.java b/runelite-client/src/main/java/net/runelite/client/plugins/chatnotifications/ChatNotificationsPlugin.java
index 0f8c5f1847..00b7cc80d4 100644
--- a/runelite-client/src/main/java/net/runelite/client/plugins/chatnotifications/ChatNotificationsPlugin.java
+++ b/runelite-client/src/main/java/net/runelite/client/plugins/chatnotifications/ChatNotificationsPlugin.java
@@ -82,7 +82,7 @@ public class ChatNotificationsPlugin extends Plugin
 	private Pattern highlightMatcher = null;
 
 	// Private message cache used to avoid duplicate notifications from ChatHistory.
-	private Set<Integer> privateMessageHashes = new HashSet<>();
+	private final Set<Integer> privateMessageHashes = new HashSet<>();
 
 	private boolean highlightOwnName;
 	private String highlightWordsString;