diff --git a/src/obj.rs b/src/obj.rs
index 2b34aca..bf7239c 100644
--- a/src/obj.rs
+++ b/src/obj.rs
@@ -147,8 +147,8 @@ pub trait Object: Debug + Display + Any + Trace {
         let mut ty = self.ty();
         loop {
             let vtable_entry =
-                with_obj_downcast_mut(ty.clone(), |ty: &mut Ty| ty.vtable().get(name).cloned())
-                    .map(|vtable_entry| {
+                with_obj_downcast(ty.clone(), |ty: &Ty| ty.vtable().get(name).cloned()).map(
+                    |vtable_entry| {
                         let ptr = if obj_is_inst::<BuiltinFunction>(&vtable_entry)
                             || obj_is_inst::<UserFunction>(&vtable_entry)
                         {
@@ -161,7 +161,8 @@ pub trait Object: Debug + Display + Any + Trace {
                         // floating around.
                         //self.set_attr(name, ptr.clone());
                         ptr
-                    });
+                    },
+                );
             if vtable_entry.is_some() {
                 return vtable_entry;
             }
@@ -261,6 +262,10 @@ impl Debug for Obj {
 
 impl Object for Obj {
     fn instantiate(&mut self) {
+        if self.get_attr("__ty__").is_none() {
+            let ty = BUILTINS.with_borrow(|builtins| builtins.get("Obj").expect("no Obj").clone());
+            self.set_attr("__ty__", ty);
+        }
         self.is_instantiated = true;
     }