diff --git a/nhf/parts/joints.py b/nhf/parts/joints.py
index 4245eff..f375919 100644
--- a/nhf/parts/joints.py
+++ b/nhf/parts/joints.py
@@ -317,7 +317,7 @@ class TorsionJoint:
         if not rider_slot_begin:
             rider_slot_begin = self.rider_slot_begin
         def slot(loc):
-            wire = Cq.Wire.makePolygon(self._slot_polygon(flip=False))
+            wire = Cq.Wire.makePolygon(self._slot_polygon(flip=False)).close()
             face = Cq.Face.makeFromWires(wire)
             return face.located(loc)
         wall_profile = (