diff --git a/screening/source/java/ch/systemsx/cisd/openbis/plugin/screening/shared/api/v1/dto/PlateIdentifier.java b/screening/source/java/ch/systemsx/cisd/openbis/plugin/screening/shared/api/v1/dto/PlateIdentifier.java
index 610225728968bc63b740921a3dff0424e76a913e..5f6ab00afc961177c94fd39988ad3727bdad3cda 100644
--- a/screening/source/java/ch/systemsx/cisd/openbis/plugin/screening/shared/api/v1/dto/PlateIdentifier.java
+++ b/screening/source/java/ch/systemsx/cisd/openbis/plugin/screening/shared/api/v1/dto/PlateIdentifier.java
@@ -11,7 +11,7 @@ public class PlateIdentifier implements Serializable
 {
     private static final long serialVersionUID = 1L;
 
-    private String plateCode, spaceCodeOrNull;
+    private final String plateCode, spaceCodeOrNull;
 
     public PlateIdentifier(String plateCode, String spaceCodeOrNull)
     {