diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index 741b2a9..a5e5a69 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -7543,7 +7543,11 @@ public sealed class GameWindow : IDisposable
         // we always want it animated in player mode.
         if (!_animatedEntities.TryGetValue(pe.Id, out var ae))
         {
-            var setup = _dats.Get<DatReaderWriter.DBObjs.Setup>(pe.SourceGfxObjOrSetupId);
+            // A.5 T10: lock around _dats.Get — worker thread may be
+            // building a landblock mesh concurrently. DatBinReader's
+            // shared buffer position would corrupt without serialization.
+            DatReaderWriter.DBObjs.Setup? setup;
+            lock (_datLock) { setup = _dats.Get<DatReaderWriter.DBObjs.Setup>(pe.SourceGfxObjOrSetupId); }
             if (setup is null) return;
             _physicsDataCache.CacheSetup(pe.SourceGfxObjOrSetupId, setup);
 
@@ -8564,7 +8568,10 @@ public sealed class GameWindow : IDisposable
         // 0.4 m fallbacks.
         if (_dats is not null && (playerEntity.SourceGfxObjOrSetupId & 0xFF000000u) == 0x02000000u)
         {
-            var playerSetup = _dats.Get<DatReaderWriter.DBObjs.Setup>(playerEntity.SourceGfxObjOrSetupId);
+            // A.5 T10: lock around _dats.Get — worker thread may be
+            // building a landblock mesh concurrently.
+            DatReaderWriter.DBObjs.Setup? playerSetup;
+            lock (_datLock) { playerSetup = _dats.Get<DatReaderWriter.DBObjs.Setup>(playerEntity.SourceGfxObjOrSetupId); }
             if (playerSetup is not null)
                 _physicsDataCache.CacheSetup(playerEntity.SourceGfxObjOrSetupId, playerSetup);
             _playerController.StepUpHeight = (playerSetup is not null && playerSetup.StepUpHeight > 0f)