diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusBackgroundLoopTokenLifetimeTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusBackgroundLoopTokenLifetimeTests.cs
index 91cee0010..7a4e2478a 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusBackgroundLoopTokenLifetimeTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusBackgroundLoopTokenLifetimeTests.cs
@@ -138,7 +138,7 @@ public sealed class ModbusBackgroundLoopTokenLifetimeTests
 
         gate.ArmedSchedules.ShouldBe(1, "exactly one task (the background loop) must be scheduled by initialize; anything else is ambiguous");
         var probeTask = Field<Task>(drv, taskField);
-        probeTask.ShouldNotBeNull();
+        Assert.NotNull(probeTask); // not Shouldly: its Task overload would be an un-awaited async assertion
 
         await gate.Started.Task.WaitAsync(Bound, TestContext.Current.CancellationToken);
         probeTask.IsCompleted.ShouldBeFalse("the worker must be parked after its start check, before its delegate runs");