diff --git a/services/acl/Acl.Application/InMemoryDefaultFillStore.cs b/services/acl/Acl.Application/InMemoryDefaultFillStore.cs
index de4ede6..b6f5d5a 100644
--- a/services/acl/Acl.Application/InMemoryDefaultFillStore.cs
+++ b/services/acl/Acl.Application/InMemoryDefaultFillStore.cs
@@ -14,6 +14,7 @@ public sealed class InMemoryDefaultFillStore(DefaultFillSettings seed) : IDefaul
 
     public void Update(DefaultFillSettings settings)
     {
-        // ponytail: red stub — real assignment lands in the green commit.
+        ArgumentNullException.ThrowIfNull(settings);
+        lock (_gate) _current = settings;
     }
 }