diff --git a/ocpp-v1_6/src/main/java/eu/chargetime/ocpp/model/core/StopTransactionRequest.java b/ocpp-v1_6/src/main/java/eu/chargetime/ocpp/model/core/StopTransactionRequest.java
index 1f46b0c62..58f707a37 100644
--- a/ocpp-v1_6/src/main/java/eu/chargetime/ocpp/model/core/StopTransactionRequest.java
+++ b/ocpp-v1_6/src/main/java/eu/chargetime/ocpp/model/core/StopTransactionRequest.java
@@ -103,7 +103,7 @@ public String getIdTag() {
    */
   @XmlElement
   public void setIdTag(String idTag) {
-    if (!ModelUtil.validate(idTag, 20)) {
+    if (idTag != null && !ModelUtil.validate(idTag, 20)) {
       throw new PropertyConstraintException(idTag.length(), "Exceeded limit of 20 chars");
     }
 
diff --git a/ocpp-v1_6/src/test/java/eu/chargetime/ocpp/model/test/StopTransactionRequestTest.java b/ocpp-v1_6/src/test/java/eu/chargetime/ocpp/model/test/StopTransactionRequestTest.java
index 2f2734ad1..559ff95fb 100644
--- a/ocpp-v1_6/src/test/java/eu/chargetime/ocpp/model/test/StopTransactionRequestTest.java
+++ b/ocpp-v1_6/src/test/java/eu/chargetime/ocpp/model/test/StopTransactionRequestTest.java
@@ -75,6 +75,20 @@ public void setIdTag_moreThan20Chars_throwsPropertyConstraintException() {
     request.setIdTag(aString(42));
   }
 
+  @Test
+  public void setIdTag_nullGiven_doesNotRaiseException() {
+    request.setIdTag(null);
+    Assert.assertNull(request.getIdTag());
+  }
+  @Test
+  public void setIdTag_nullGiven_passesValidation() {
+    request.setMeterStop(2);
+    request.setTransactionId(5);
+    request.setTimestamp(ZonedDateTime.now());
+    request.setIdTag(null);
+    Assert.assertTrue(request.validate());
+  }
+
   @Test
   public void setMeterStop_anInteger_meterStopIsSet() {
     // Given