diff --git a/src/r_bsp.c b/src/r_bsp.c
index 052798d1..99c4ecfb 100644
--- a/src/r_bsp.c
+++ b/src/r_bsp.c
@@ -383,6 +383,11 @@ static void R_AddLine (seg_t *line)
   //      should already be interpolated.
   R_MaybeInterpolateSector(backsector);
 
+  if (backsector->heightsec != -1)
+  {
+    R_MaybeInterpolateSector(&sectors[backsector->heightsec]);
+  }
+
   // killough 3/8/98, 4/4/98: hack for invisible ceilings / deep water
   backsector = R_FakeFlat(backsector, &tempsec, NULL, NULL, true);