diff --git a/vm/src/builtins/memory.rs b/vm/src/builtins/memory.rs
index cf6004e097..a34d8db3ac 100644
--- a/vm/src/builtins/memory.rs
+++ b/vm/src/builtins/memory.rs
@@ -876,7 +876,7 @@ struct CastArgs {
     #[pyarg(any)]
     format: PyStrRef,
     #[pyarg(any, optional)]
-    shape: OptionalArg<PyObjectRef>,
+    shape: OptionalArg<Either<PyTupleRef, PyListRef>>,
 }
 
 enum SubscriptNeedle {