diff --git a/src/main/java/de/neemann/digital/draw/model/InverterConfig.java b/src/main/java/de/neemann/digital/draw/model/InverterConfig.java
index 46286b991..0a1980639 100644
--- a/src/main/java/de/neemann/digital/draw/model/InverterConfig.java
+++ b/src/main/java/de/neemann/digital/draw/model/InverterConfig.java
@@ -17,7 +17,7 @@ import java.util.Objects;
  */
 public final class InverterConfig implements HGSMap {
 
-    private HashSet<String> inputs;
+    private final HashSet<String> inputs;
 
     private InverterConfig(HashSet<String> inputs) {
         this.inputs = inputs;