diff --git a/tests/FullstackTemplate.FunctionalTests/TestingWebApplicationFactory.cs b/tests/FullstackTemplate.FunctionalTests/TestingWebApplicationFactory.cs
index 4331a20..91c3def 100644
--- a/tests/FullstackTemplate.FunctionalTests/TestingWebApplicationFactory.cs
+++ b/tests/FullstackTemplate.FunctionalTests/TestingWebApplicationFactory.cs
@@ -25,6 +25,10 @@ public sealed class TestingWebApplicationFactory : WebApplicationFactory<Program
 {
     private readonly PostgresTestDatabase _database = new();
 
+    // WebApplicationFactory adds each client to a List with no lock, and the tests create their clients in parallel.
+    // Two adds at the same time can lose a client or leave a null in the list, and DisposeAsync then throws.
+    private readonly Lock _clientLock = new();
+
     /// <summary>
     /// The machine client that the test server lets manage all tenants. It also reads the rows of all tenants.
     /// </summary>
@@ -125,6 +129,12 @@ public async Task InitializeAsync()
         Instance = this;
     }
 
+    public new HttpClient CreateClient()
+    {
+        lock (_clientLock)
+            return base.CreateClient();
+    }
+
     public override async ValueTask DisposeAsync()
     {
         await base.DisposeAsync();