diff --git a/src/AcDream.App/Streaming/LandblockStreamer.cs b/src/AcDream.App/Streaming/LandblockStreamer.cs
index 6b08095..a3416de 100644
--- a/src/AcDream.App/Streaming/LandblockStreamer.cs
+++ b/src/AcDream.App/Streaming/LandblockStreamer.cs
@@ -75,20 +75,27 @@ public sealed class LandblockStreamer : IDisposable
     }
 
     /// <summary>
-    /// Activate the dedicated background worker thread. Idempotent: calling
-    /// <see cref="Start"/> more than once has no effect.
+    /// Activate the dedicated background worker thread. Idempotent and
+    /// thread-safe: concurrent callers will only spawn one worker; subsequent
+    /// calls are no-ops. Atomic via <see cref="Interlocked.CompareExchange{T}(ref T, T, T)"/>.
     /// </summary>
     public void Start()
     {
         if (System.Threading.Volatile.Read(ref _disposed) != 0)
             throw new ObjectDisposedException(nameof(LandblockStreamer));
-        if (_worker != null) return;
-        _worker = new Thread(WorkerLoop)
+
+        // A.5 T10-T12 follow-up: atomically install the worker so concurrent
+        // Start() callers don't both pass the null check and spawn duplicate
+        // threads. Construct the candidate; CAS it into _worker; if we lost
+        // the race, the candidate goes unstarted and is GCed.
+        var candidate = new Thread(WorkerLoop)
         {
             IsBackground = true,
             Name = "acdream.streaming.worker",
         };
-        _worker.Start();
+        if (Interlocked.CompareExchange(ref _worker, candidate, null) == null)
+            candidate.Start();
+        // else: another caller won the race; their thread is running.
     }
 
     /// <summary>
diff --git a/tests/AcDream.Core.Tests/Streaming/LandblockStreamerTests.cs b/tests/AcDream.Core.Tests/Streaming/LandblockStreamerTests.cs
index 2e11804..7c5291c 100644
--- a/tests/AcDream.Core.Tests/Streaming/LandblockStreamerTests.cs
+++ b/tests/AcDream.Core.Tests/Streaming/LandblockStreamerTests.cs
@@ -66,6 +66,39 @@ public class LandblockStreamerTests
         Assert.IsType<LandblockStreamResult.Failed>(result);
     }
 
+    [Fact]
+    public async Task Load_WhenBuildMeshReturnsNull_ReportsFailed()
+    {
+        // Phase A.5 T10-T12 follow-up: the mesh-build factory may return
+        // null (e.g., LandBlock dat missing or corrupt). The worker must
+        // emit Failed in that case instead of constructing Loaded with a
+        // null MeshData (which would NRE downstream).
+        var stubLandblock = new LoadedLandblock(
+            0xABCDFFFEu,
+            new LandBlock(),
+            System.Array.Empty<WorldEntity>());
+
+        using var streamer = new LandblockStreamer(
+            loadLandblock: _ => stubLandblock,
+            buildMeshOrNull: (_, _) => null);   // mesh-build returns null
+
+        streamer.Start();
+        streamer.EnqueueLoad(0xABCDFFFEu);
+
+        LandblockStreamResult? result = null;
+        for (int i = 0; i < SpinMaxIterations && result is null; i++)
+        {
+            var drained = streamer.DrainCompletions(LandblockStreamer.DefaultDrainBatchSize);
+            if (drained.Count > 0) result = drained[0];
+            else await Task.Delay(SpinStepMs);
+        }
+
+        Assert.NotNull(result);
+        var failed = Assert.IsType<LandblockStreamResult.Failed>(result);
+        Assert.Equal(0xABCDFFFEu, failed.LandblockId);
+        Assert.Contains("mesh", failed.Error, System.StringComparison.OrdinalIgnoreCase);
+    }
+
     [Fact]
     public async Task Load_WhenLoaderThrows_ReportsFailedWithMessage()
     {
diff --git a/tests/AcDream.Core.Tests/Streaming/StreamingControllerTests.cs b/tests/AcDream.Core.Tests/Streaming/StreamingControllerTests.cs
index bafe59a..cb79116 100644
--- a/tests/AcDream.Core.Tests/Streaming/StreamingControllerTests.cs
+++ b/tests/AcDream.Core.Tests/Streaming/StreamingControllerTests.cs
@@ -78,7 +78,13 @@ public class StreamingControllerTests
         // Entities (positional record). Adjust if the first positional arg
         // name differs.
         var lb = new LoadedLandblock(0x32320FFEu, new LandBlock(), System.Array.Empty<WorldEntity>());
-        fake.Pending.Enqueue(new LandblockStreamResult.Loaded(0x32320FFEu, LandblockStreamTier.Near, lb, MeshData: default!));
+        // A.5 T10-T12 follow-up: use a real empty mesh instance instead of
+        // default! so any future test that flows MeshData through the apply
+        // callback gets a non-null reference to inspect rather than an NRE.
+        var stubMesh = new AcDream.Core.Terrain.LandblockMeshData(
+            System.Array.Empty<AcDream.Core.Terrain.TerrainVertex>(),
+            System.Array.Empty<uint>());
+        fake.Pending.Enqueue(new LandblockStreamResult.Loaded(0x32320FFEu, LandblockStreamTier.Near, lb, stubMesh));
 
         controller.Tick(50, 50);