diff --git a/src/AcDream.Core/World/Cells/CellGraph.cs b/src/AcDream.Core/World/Cells/CellGraph.cs
index 80ee9ac..8f19e59 100644
--- a/src/AcDream.Core/World/Cells/CellGraph.cs
+++ b/src/AcDream.Core/World/Cells/CellGraph.cs
@@ -49,5 +49,10 @@ public sealed class CellGraph
         return LandCell.Synthesize(id, t.Terrain, t.Origin, idx / 8, idx % 8);
     }
 
+    /// <summary>
+    /// Resolve the cell on the far side of a portal — retail CCellPortal::GetOtherCell =
+    /// GetVisible(other_cell_id). <paramref name="cell"/> documents the traversal source and is
+    /// reserved for the OtherCellPtr neighbor cache (Stage 3); the lookup keys only on the portal.
+    /// </summary>
     public ObjCell? Neighbor(ObjCell cell, in CellPortal portal) => GetVisible(portal.OtherCellId);
 }
diff --git a/src/AcDream.Core/World/Cells/EnvCell.cs b/src/AcDream.Core/World/Cells/EnvCell.cs
index 10a0407..cb24f03 100644
--- a/src/AcDream.Core/World/Cells/EnvCell.cs
+++ b/src/AcDream.Core/World/Cells/EnvCell.cs
@@ -66,8 +66,9 @@ public sealed class EnvCell : ObjCell
         }
 
         uint lbPrefix = id & 0xFFFF0000u;
-        var stab = new List<uint>(datCell.VisibleCells.Count);
-        foreach (var low in datCell.VisibleCells) stab.Add(lbPrefix | low);
+        var stab = new List<uint>();
+        if (datCell.VisibleCells is not null)                       // match BuildLoadedCell:5701 null guard
+            foreach (var low in datCell.VisibleCells) stab.Add(lbPrefix | low);
         bool seenOutside = datCell.Flags.HasFlag(EnvCellFlags.SeenOutside);
 
         return new EnvCell(id, worldTransform, inverse, min, max, portals, stab,