diff --git a/src/Quox/Syntax/Subst.idr b/src/Quox/Syntax/Subst.idr
index 47f0b8c..2658e0a 100644
--- a/src/Quox/Syntax/Subst.idr
+++ b/src/Quox/Syntax/Subst.idr
@@ -65,8 +65,8 @@ shift by = Shift $ fromNat by
 infixl 9 .
 public export
 (.) : CanSubst1 f => Subst f from mid -> Subst f mid to -> Subst f from to
+Shift by      . Shift bz   = Shift $ by . bz
 Shift SZ      . ph         = ph
-Shift (SS by) . Shift bz   = Shift $ SS by . bz
 Shift (SS by) . (t ::: th) = Shift by . th
 (t ::: th)    . ph         = (t // ph) ::: (th . ph)