diff --git a/src/obj.rs b/src/obj.rs
index 08a1876..82324d2 100644
--- a/src/obj.rs
+++ b/src/obj.rs
@@ -256,20 +256,24 @@ pub trait Obj: Debug + Display + Any + Trace {
 
         let mut type_inst = self.type_inst();
         loop {
-            with_obj_downcast_mut(type_inst.clone(), |type_inst: &mut TypeInst| {
-                type_inst.vtable.get(name).cloned()
-            })
-            .map(|vtable_entry| {
-                let ptr = if obj_is_inst::<BuiltinFunctionInst>(&vtable_entry)
-                    || obj_is_inst::<UserFunctionInst>(&vtable_entry)
-                {
-                    MethodInst::create(method_ty.clone(), self_ptr.clone(), vtable_entry)
-                } else {
-                    vtable_entry
-                };
-                self.set_attr(name, ptr.clone());
-                ptr
-            })?;
+            let vtable_entry =
+                with_obj_downcast_mut(type_inst.clone(), |type_inst: &mut TypeInst| {
+                    type_inst.vtable.get(name).cloned()
+                })
+                .map(|vtable_entry| {
+                    let ptr = if obj_is_inst::<BuiltinFunctionInst>(&vtable_entry)
+                        || obj_is_inst::<UserFunctionInst>(&vtable_entry)
+                    {
+                        MethodInst::create(method_ty.clone(), self_ptr.clone(), vtable_entry)
+                    } else {
+                        vtable_entry
+                    };
+                    self.set_attr(name, ptr.clone());
+                    ptr
+                });
+            if vtable_entry.is_some() {
+                return vtable_entry;
+            }
             let type_inst_copy = type_inst.borrow().type_inst();
             if type_inst.borrow().equals(&*type_inst_copy.borrow()) {
                 return None;
@@ -1158,6 +1162,7 @@ fn test_obj_equals() {
     assert!(obj2.borrow().equals(&*obj1.borrow()));
 }
 
+#[test]
 fn test_obj_vtable() {
     let mut builtins = HashMap::new();
     init_types(&mut builtins);