diff --git a/stimpl/runtime.py b/stimpl/runtime.py
index 69910c8..e78736a 100644
--- a/stimpl/runtime.py
+++ b/stimpl/runtime.py
@@ -50,7 +50,7 @@ def __repr__(self) -> str:
 def evaluate(expression: Expr, state: State) -> Tuple[Optional[Any], Type, State]:
     match expression:
         case Ren():
-            return ((), Unit(), state)
+            return (None, Unit(), state)
 
         case IntLiteral(literal=l):
             return (l, Integer(), state)