diff --git a/README.md b/README.md
index 2c8a4d7..36284f7 100644
--- a/README.md
+++ b/README.md
@@ -134,6 +134,7 @@ pylyzer converts Python ASTs to Erg ASTs and passes them to Erg's type checker.
   * [ ] others
 * [x] type assertion (`typing.cast`)
 * [x] type narrowing (`is`, `isinstance`)
+* [ ] `pyi` (stub) files support
 
 ---
 
diff --git a/crates/py2erg/convert.rs b/crates/py2erg/convert.rs
index 1a7daa7..2ce5734 100644
--- a/crates/py2erg/convert.rs
+++ b/crates/py2erg/convert.rs
@@ -947,7 +947,22 @@ impl ASTConverter {
                 self.convert_ident_type_spec(name.id.to_string(), name.location())
             }
             py_ast::Expr::Constant(cons) => {
-                self.convert_ident_type_spec("NoneType".into(), cons.location())
+                if cons.value.is_none() {
+                    self.convert_ident_type_spec("NoneType".into(), cons.location())
+                } else if let Some(name) = cons.value.as_str() {
+                    self.convert_ident_type_spec(name.into(), cons.location())
+                } else {
+                    let err = CompileError::syntax_error(
+                        self.cfg.input.clone(),
+                        line!() as usize,
+                        pyloc_to_ergloc(cons.range()),
+                        self.cur_namespace(),
+                        format!("{:?} is not a type", cons.value),
+                        None,
+                    );
+                    self.errs.push(err);
+                    Self::gen_dummy_type_spec(cons.location())
+                }
             }
             py_ast::Expr::Attribute(attr) => {
                 let namespace = Box::new(self.convert_expr(*attr.value));
diff --git a/tests/class.py b/tests/class.py
index 40ae684..ee31761 100644
--- a/tests/class.py
+++ b/tests/class.py
@@ -19,6 +19,8 @@ class C:
         return self.x
     def id(self) -> Self:
         return self
+    def id2(self) -> "C":
+        return self
 
 c = C(1, 2)
 assert c.x == 1