diff --git a/source/core/gamefuncs.h b/source/core/gamefuncs.h
index 6bc5af57c..0b1f4b641 100644
--- a/source/core/gamefuncs.h
+++ b/source/core/gamefuncs.h
@@ -139,9 +139,8 @@ void vertexscan(walltype* startwall, func mark)
 	while (true)
 	{
 		auto thelastwall = wal->lastWall();
-		// thelastwall can be null here if the map is bogus.
-		if (!thelastwall || !thelastwall->twoSided()) break;
-
+		// thelastwall can be null here if the map is bogus. 
+		if (!thelastwall || thelastwall->nextwall < 0) break;
 		wal = thelastwall->nextWall();
 		if (walbitmap.Check(wall.IndexOf(wal))) break;
 		mark(wal);
diff --git a/source/core/maploader.cpp b/source/core/maploader.cpp
index 237af13b7..71ed37d92 100644
--- a/source/core/maploader.cpp
+++ b/source/core/maploader.cpp
@@ -831,7 +831,11 @@ void setWallSectors()
 				wal.nextsector = wal.nextWall()->sector;
 			}
 		}
-		else wal.nextwall = -1;
+		else
+		{
+			wal.nextwall = -1;
+			wal.nextsector = -1;
+		}
 	}
 
 }