diff --git a/src/AcDream.App/Rendering/Wb/WbMeshAdapter.cs b/src/AcDream.App/Rendering/Wb/WbMeshAdapter.cs
index 0d8dee8..9893e31 100644
--- a/src/AcDream.App/Rendering/Wb/WbMeshAdapter.cs
+++ b/src/AcDream.App/Rendering/Wb/WbMeshAdapter.cs
@@ -175,7 +175,30 @@ public sealed class WbMeshAdapter : IDisposable, IWbMeshAdapter
                     }
                     else if (t.IsCompletedSuccessfully && t.Result is null)
                     {
-                        Console.WriteLine($"[indoor-upload] NULL_RESULT cellId=0x{cellId:X8}");
+                        // Phase 2 cause-narrowing: WB's PrepareMeshData can return
+                        // null for several reasons (ResolveId empty / TryGet<EnvCell>
+                        // failed / type Unknown). Cross-check against acdream's own
+                        // DatCollection — if WE find the cell but WB doesn't, the
+                        // divergence is between dat readers, not a missing record.
+                        bool ourCellFound = false;
+                        try
+                        {
+                            ourCellFound = _dats?.Cell.TryGet<DatReaderWriter.DBObjs.EnvCell>(
+                                (uint)cellId, out _) ?? false;
+                        }
+                        catch { /* swallow — this is best-effort diagnostic */ }
+
+                        int wbResolveCount = -1;
+                        try
+                        {
+                            wbResolveCount = _wbDats?.ResolveId((uint)cellId).Count() ?? -1;
+                        }
+                        catch { /* swallow — best-effort */ }
+
+                        Console.WriteLine(
+                            $"[indoor-upload] NULL_RESULT cellId=0x{cellId:X8} " +
+                            $"ourCellDb.TryGet={ourCellFound} " +
+                            $"wbResolveId.Count={wbResolveCount}");
                     }
                 }, TaskScheduler.Default);
             }