diff --git a/modules/book/src/main/java/me/ag2s/epublib/domain/Resources.java b/modules/book/src/main/java/me/ag2s/epublib/domain/Resources.java
index 33cda3253..aaa233e4d 100644
--- a/modules/book/src/main/java/me/ag2s/epublib/domain/Resources.java
+++ b/modules/book/src/main/java/me/ag2s/epublib/domain/Resources.java
@@ -31,7 +31,7 @@ public class Resources implements Serializable {
 
     private Map<String, Resource> resources = new HashMap<>();
 
-    private Map<String, Resource> resourcesById = new HashMap<>();
+    private final Map<String, Resource> resourcesById = new HashMap<>();
 
     /**
      * Adds a resource to the resources.