diff --git a/li.strolch.model/src/main/java/li/strolch/model/json/FromFlatJsonVisitor.java b/li.strolch.model/src/main/java/li/strolch/model/json/FromFlatJsonVisitor.java
index 8771bb442..283ae3f36 100644
--- a/li.strolch.model/src/main/java/li/strolch/model/json/FromFlatJsonVisitor.java
+++ b/li.strolch.model/src/main/java/li/strolch/model/json/FromFlatJsonVisitor.java
@@ -36,8 +36,8 @@ import li.strolch.utils.dbc.DBC;
 public class FromFlatJsonVisitor implements StrolchRootElementVisitor<Void> {
 
 	private Set<String> ignoredBagTypes;
-	private MapOfSets<String, String> ignoredKeys;
-	private MapOfSets<String, String> optionalKeys;
+	private final MapOfSets<String, String> ignoredKeys;
+	private final MapOfSets<String, String> optionalKeys;
 	private MapOfSets<String, String> nonEmptyParameters;
 
 	private JsonObject srcObject;