diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/GalaxyDriverAlarmSourceTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/GalaxyDriverAlarmSourceTests.cs
index dd523b0e..15e88571 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/GalaxyDriverAlarmSourceTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/GalaxyDriverAlarmSourceTests.cs
@@ -192,6 +192,7 @@ public sealed class GalaxyDriverAlarmSourceTests
         ack.Calls.Count.ShouldBe(2);
         ack.Calls[0].AlarmRef.ShouldBe("Tank01.Level.HiHi");
         ack.Calls[0].Comment.ShouldBe("shift handover");
+        ack.Calls[0].Operator.ShouldBe(string.Empty); // null OperatorUser must fall back to empty string
         ack.Calls[1].AlarmRef.ShouldBe("Tank02.Level.HiHi");
     }