diff --git a/src/Syntax.hs b/src/Syntax.hs
index fd13992bb..ec57fde72 100644
--- a/src/Syntax.hs
+++ b/src/Syntax.hs
@@ -167,3 +167,6 @@ instance Listable2 Syntax where
 
 instance Listable leaf => Listable1 (Syntax leaf) where
   liftTiers = liftTiers2 tiers
+
+instance (Listable leaf, Listable recur) => Listable (Syntax leaf recur) where
+  tiers = tiers1