diff --git a/src/p_setup.cpp b/src/p_setup.cpp
index 5efdac413..b19b64240 100644
--- a/src/p_setup.cpp
+++ b/src/p_setup.cpp
@@ -1967,8 +1967,14 @@ void P_LoadLineDefs (MapData * map)
 		}
 		else
 		{
-			if (LittleShort(mld->sidenum[0]) != NO_INDEX)
-				sidecount++;
+			// 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)
+			{
+				Printf("Line %d has no first side.\n", i);
+				mld->sidenum[0] = 0;
+			}
+			sidecount++;
 			if (LittleShort(mld->sidenum[1]) != NO_INDEX)
 				sidecount++;
 			linemap[i] = i+skipped;
@@ -2037,8 +2043,14 @@ void P_LoadLineDefs2 (MapData * map)
 		}
 		else
 		{
-			if (LittleShort(mld->sidenum[0]) != NO_INDEX)
-				sidecount++;
+			// 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)
+			{
+				Printf("Line %d has no first side.\n", i);
+				mld->sidenum[0] = 0;
+			}
+			sidecount++;
 			if (LittleShort(mld->sidenum[1]) != NO_INDEX)
 				sidecount++;
 			linemap[i] = i+skipped;
diff --git a/src/p_udmf.cpp b/src/p_udmf.cpp
index 33fb027a7..6cbfccbef 100644
--- a/src/p_udmf.cpp
+++ b/src/p_udmf.cpp
@@ -850,6 +850,11 @@ struct UDMFParser
 		{
 			ld->Alpha = FRACUNIT * 3/4;
 		}
+		if (ld->sidedef[0] == NULL)
+		{
+			ld->sidedef[0] = (side_t*)(intptr_t)(1);
+			Printf("Line %d has no first side.\n", index);
+		}
 	}
 
 	//===========================================================================