diff --git a/src/Quox/Syntax/Dim.idr b/src/Quox/Syntax/Dim.idr
index f05f64e..6dd0e90 100644
--- a/src/Quox/Syntax/Dim.idr
+++ b/src/Quox/Syntax/Dim.idr
@@ -61,6 +61,11 @@ prettyDSubst th =
 
 export FromVar Dim where fromVar = B
 
+export
+CanShift Dim where
+  K e // _  = K e
+  B i // by = B (i // by)
+
 export
 CanSubst Dim Dim where
   K e // _  = K e