diff --git a/crates/erg_compiler/context/inquire.rs b/crates/erg_compiler/context/inquire.rs
index a435a02e..f91b17bf 100644
--- a/crates/erg_compiler/context/inquire.rs
+++ b/crates/erg_compiler/context/inquire.rs
@@ -859,7 +859,8 @@ impl Context {
                         return Triple::Err(errs);
                     }
                     drop(list);
-                    let res = self.sub_unify(obj.ref_t(), self_t, obj, Some(&"self".into()));
+                    let res =
+                        self.self_unify(obj.ref_t(), self_t, &vi.t, obj, Some(&"self".into()));
                     if DEBUG_MODE {
                         res.unwrap();
                     }
@@ -2150,7 +2151,13 @@ impl Context {
         let non_default_params = if is_method_call {
             let mut non_default_params = subr.non_default_params.iter();
             let self_pt = non_default_params.next().unwrap();
-            if let Err(mut es) = self.sub_unify(obj.ref_t(), self_pt.typ(), obj, self_pt.name()) {
+            if let Err(mut es) = self.self_unify(
+                obj.ref_t(),
+                self_pt.typ(),
+                &Type::Subr(subr.clone()),
+                obj,
+                self_pt.name(),
+            ) {
                 errs.append(&mut es);
             }
             passed_params.insert("self".into());
diff --git a/crates/erg_compiler/context/instantiate.rs b/crates/erg_compiler/context/instantiate.rs
index 9b5684d7..a7433a10 100644
--- a/crates/erg_compiler/context/instantiate.rs
+++ b/crates/erg_compiler/context/instantiate.rs
@@ -1037,7 +1037,13 @@ impl Context {
                 let mut tmp_tv_cache = TyVarCache::new(self.level, self);
                 let ty = self.instantiate_t_inner(*quant, &mut tmp_tv_cache, callee)?;
                 if let Some(self_t) = ty.self_t() {
-                    self.sub_unify(callee.ref_t(), self_t, callee, Some(&Str::ever("self")))?;
+                    self.self_unify(
+                        callee.ref_t(),
+                        self_t,
+                        &ty,
+                        callee,
+                        Some(&Str::ever("self")),
+                    )?;
                 }
                 if DEBUG_MODE && ty.has_qvar() {
                     panic!("{ty} has qvar")
@@ -1050,9 +1056,10 @@ impl Context {
                 match &t {
                     Type::Subr(subr) => {
                         if let Some(self_t) = subr.self_t() {
-                            self.sub_unify(
+                            self.self_unify(
                                 callee.ref_t(),
                                 self_t,
+                                &t,
                                 callee,
                                 Some(&Str::ever("self")),
                             )?;
@@ -1061,9 +1068,10 @@ impl Context {
                     Type::And(tys, _) => {
                         for ty in tys {
                             if let Some(self_t) = ty.self_t() {
-                                self.sub_unify(
+                                self.self_unify(
                                     callee.ref_t(),
                                     self_t,
+                                    ty,
                                     callee,
                                     Some(&Str::ever("self")),
                                 )?;
diff --git a/crates/erg_compiler/context/unify.rs b/crates/erg_compiler/context/unify.rs
index 85b5ac4a..c6efa731 100644
--- a/crates/erg_compiler/context/unify.rs
+++ b/crates/erg_compiler/context/unify.rs
@@ -2267,6 +2267,33 @@ impl Context {
         unifier.sub_unify(maybe_sub, maybe_super)
     }
 
+    pub(crate) fn self_unify(
+        &self,
+        sub_self: &Type,
+        super_self: &Type,
+        subr: &Type,
+        loc: &impl Locational,
+        param_name: Option<&Str>,
+    ) -> TyCheckResult<()> {
+        let unifier = Unifier::new(self, loc, None, false, param_name.cloned());
+        unifier.sub_unify(sub_self, super_self)?;
+        let super_self = match super_self {
+            Type::Ref(inner) => inner,
+            Type::RefMut { before, .. } => before,
+            _ => super_self,
+        };
+        let sub_self = sub_self.derefine();
+        if self.subtype_of(super_self, &sub_self) {
+            if let Some(return_t) = subr.return_t() {
+                if return_t.has_no_unbound_var() && !return_t.contains_type(&sub_self) {
+                    // callee.ref_t() == self_t
+                    let _ = unifier.sub_unify(super_self, &sub_self);
+                }
+            }
+        }
+        Ok(())
+    }
+
     pub(crate) fn sub_unify_with_coercion(
         &self,
         maybe_sub: &Type,
diff --git a/tests/should_err/mut_list.er b/tests/should_err/mut_list.er
index 19103446..52a15ab4 100644
--- a/tests/should_err/mut_list.er
+++ b/tests/should_err/mut_list.er
@@ -14,3 +14,9 @@ i_s.push! None # ERR
 
 _: List!(Int, _) = !["a"] # ERR
 _: List!(Int, 1) = ![1, 2] # ERR
+
+C! = Class(List!(List!(Int)))
+C!.
+    append! ref! self =
+        self::base.insert! 0, !["a"] # ERR
+        self::base.insert! 0, [1] # ERR
diff --git a/tests/test.rs b/tests/test.rs
index cc90e6ae..a3342939 100644
--- a/tests/test.rs
+++ b/tests/test.rs
@@ -766,7 +766,7 @@ fn exec_mut_err() -> Result<(), ()> {
 
 #[test]
 fn exec_mut_list_err() -> Result<(), ()> {
-    expect_compile_failure("tests/should_err/mut_list.er", 0, 5)
+    expect_compile_failure("tests/should_err/mut_list.er", 0, 7)
 }
 
 #[test]