diff --git a/TombEditor/EditorActions.cs b/TombEditor/EditorActions.cs
index a595f46c8..e784f078f 100644
--- a/TombEditor/EditorActions.cs
+++ b/TombEditor/EditorActions.cs
@@ -193,10 +193,25 @@ void smoothEdit(RoomSectorPair pair, SectorEdge edge)
                         case ArrowType.CornerNW: origin = SectorEdge.XnZp; break;
                         case ArrowType.CornerSE: origin = SectorEdge.XpZn; break;
                     }
+
                     var originSector = room.GetSectorTryThroughPortal(startCoord);
-                    var originHeight = originSector.Sector.GetHeight(vertical, origin) + originSector.Room.Position.Y;
-                    for (int i = 0; i < 4; i++)
-                        corners[i] = originHeight == cornerSectors[i].Sector.GetHeight(vertical, (SectorEdge)i) + cornerSectors[i].Room.Position.Y;
+                    if (originSector.Sector == null || originSector.Room == null)
+                        Array.Fill(corners, false);
+                    else
+                    {
+                        var originHeight = originSector.Sector.GetHeight(vertical, origin) + originSector.Room.Position.Y;
+
+                        bool IsCornerValidAndAtSameHeight(RoomSectorPair cornerSector, SectorEdge edge, SectorVerticalPart cornerVertical, int referenceHeight)
+                        {
+                            if (cornerSector.Sector == null || cornerSector.Room == null)
+                                return false;
+
+                            return referenceHeight == cornerSector.Sector.GetHeight(cornerVertical, edge) + cornerSector.Room.Position.Y;
+                        }
+
+                        for (int i = 0; i < 4; i++)
+                            corners[i] = IsCornerValidAndAtSameHeight(cornerSectors[i], (SectorEdge)i, vertical, originHeight);
+                    }
                 }
 
                 // Smoothly change sectors on the corners