diff --git a/src/maploader/maploader.cpp b/src/maploader/maploader.cpp
index 04b1a058d..1d008f6b3 100644
--- a/src/maploader/maploader.cpp
+++ b/src/maploader/maploader.cpp
@@ -1675,6 +1675,7 @@ void MapLoader::LoadLineDefs (MapData * map)
 		
 	auto mldf = map->Read(ML_LINEDEFS);
 	int numlines = mldf.Size() / sizeof(maplinedef_t);
+	int numsides = map->Size(ML_SIDEDEFS) / sizeof(mapsidedef_t);
 	linemap.Resize(numlines);
 
 	// [RH] Count the number of sidedef references. This is the number of
@@ -1741,6 +1742,14 @@ void MapLoader::LoadLineDefs (MapData * map)
 		}
 #endif
 		// cph 2006/09/30 - fix sidedef errors right away.
+		for (int j=0; j < 2; j++)
+		{
+			if (LittleShort(mld->sidenum[j]) != NO_INDEX && mld->sidenum[j] >= numsides)
+			{
+				mld->sidenum[j] = 0; // dummy sidedef
+				Printf("Linedef %d has a bad sidedef\n", i);
+			}
+		}
 		// patch missing first sides instead of crashing out.
 		// Visual glitches are better than not being able to play.
 		if (LittleShort(mld->sidenum[0]) == NO_INDEX)