diff --git a/src/DiffEngine.Tests/ViewerProtocolTests.cs b/src/DiffEngine.Tests/ViewerProtocolTests.cs
index e16e3e35..e236638f 100644
--- a/src/DiffEngine.Tests/ViewerProtocolTests.cs
+++ b/src/DiffEngine.Tests/ViewerProtocolTests.cs
@@ -1053,6 +1053,9 @@ await Assert.That(await ViewerClient.SendAsync(new(ViewerVerb.List), default, se
     }
 
     [Test]
+    // Serialised: the port it releases and probes is recorded as unowned process wide, and
+    // the OS may already have handed that number to another test's owner.
+    [NotInParallel]
     public async Task AnAbsentOwnerReportsNoOwner()
     {
         ViewerServer.TryBind(0, out var server);
@@ -1119,6 +1122,8 @@ void Record(object? sender, UnobservedTaskExceptionEventArgs args)
     }
 
     [Test]
+    // Serialised for the same reason as AnAbsentOwnerReportsNoOwner.
+    [NotInParallel]
     public async Task AnAbsentOwnerIsNotAnError()
     {
         ViewerServer.TryBind(0, out var server);