diff --git a/prboom2/src/doomdata.h b/prboom2/src/doomdata.h
index f1193f6e5..4d51f03a0 100644
--- a/prboom2/src/doomdata.h
+++ b/prboom2/src/doomdata.h
@@ -143,6 +143,22 @@ typedef struct {
 //allow multiple push/switch triggers to be used on one push
 #define ML_PASSUSE      512
 
+
+// Reserved by EE
+// SoM 9/02/02: 3D Middletexture flag!
+#define ML_3DMIDTEX             1024
+
+// haleyjd 05/02/06: Although it was believed until now that a reserved line
+// flag was unnecessary, a problem with Ultimate DOOM E2M7 has disproven this
+// theory. It has roughly 1000 linedefs with 0xFE00 masked into the flags, so
+// making the next line flag reserved and using it to toggle off ALL extended
+// flags will preserve compatibility for such maps. I have been told this map
+// is one of the first ever created, so it may have something to do with that.
+#define ML_RESERVED             2048
+
+// mbf21
+#define ML_BLOCKLANDMONSTERS 4096
+
 // Sector definition, from editing.
 typedef struct {
   short floorheight;
diff --git a/prboom2/src/p_map.c b/prboom2/src/p_map.c
index b09387b05..a26a4837e 100644
--- a/prboom2/src/p_map.c
+++ b/prboom2/src/p_map.c
@@ -464,7 +464,10 @@ dboolean PIT_CheckLine (line_t* ld)
     // killough 8/9/98: monster-blockers don't affect friends
     if (
       !(tmthing->flags & MF_FRIEND || tmthing->player) &&
-      ld->flags & ML_BLOCKMONSTERS &&
+      (
+        ld->flags & ML_BLOCKMONSTERS ||
+        (mbf21 && ld->flags & ML_BLOCKLANDMONSTERS && !(tmthing->flags & MF_FLOAT))
+      ) &&
       tmthing->type != HERETIC_MT_POD
     )
       return false; // block monsters only