diff --git a/java/src/main/java/com/powsybl/dataframe/loadflow/validation/TwtValidationData.java b/java/src/main/java/com/powsybl/dataframe/loadflow/validation/TwtValidationData.java
index 434e64084b..819fde4e9b 100644
--- a/java/src/main/java/com/powsybl/dataframe/loadflow/validation/TwtValidationData.java
+++ b/java/src/main/java/com/powsybl/dataframe/loadflow/validation/TwtValidationData.java
@@ -2,6 +2,8 @@
 
 import com.powsybl.iidm.network.TwoSides;
 
+import java.util.Optional;
+
 /**
  * @author Yichen TANG <yichen.tang at rte-france.com>
  */
@@ -88,8 +90,8 @@ int getHighTapPosition() {
         return targetV;
     }
 
-    TwoSides getRegulatedSide() {
-        return regulatedSide;
+    Optional<TwoSides> getRegulatedSide() {
+        return Optional.of(regulatedSide);
     }
 
     double getV() {
diff --git a/java/src/main/java/com/powsybl/dataframe/loadflow/validation/Validations.java b/java/src/main/java/com/powsybl/dataframe/loadflow/validation/Validations.java
index 560c2fd108..4f3bc757cc 100644
--- a/java/src/main/java/com/powsybl/dataframe/loadflow/validation/Validations.java
+++ b/java/src/main/java/com/powsybl/dataframe/loadflow/validation/Validations.java
@@ -162,7 +162,7 @@ public static DataframeMapper<List<TwtValidationData>> twtsValidationMapper() {
                 .ints("low_tap_position", TwtValidationData::getLowTapPosition)
                 .ints("high_tap_position", TwtValidationData::getHighTapPosition)
                 .doubles("target_v", TwtValidationData::getTargetV)
-                .strings("regulated_side", d -> d.getRegulatedSide().name())
+                .strings("regulated_side", d -> d.getRegulatedSide().map(Enum::name).orElse(""))
                 .doubles("v", TwtValidationData::getV)
                 .booleans("connected", TwtValidationData::isConnected)
                 .booleans("main_component", TwtValidationData::isMainComponent)