diff --git a/compiler/erg_compiler/context/register.rs b/compiler/erg_compiler/context/register.rs
index 9e6cce95..5067af8e 100644
--- a/compiler/erg_compiler/context/register.rs
+++ b/compiler/erg_compiler/context/register.rs
@@ -1083,7 +1083,7 @@ impl Context {
         type_spec: ast::TypeSpec,
         call: &mut hir::Call,
     ) -> TyCheckResult<()> {
-        let target_t =
+        let cast_to =
             self.instantiate_typespec(&type_spec, None, None, RegistrationMode::Normal)?;
         let lhs = enum_unwrap!(
             call.args.get_mut_left_or_key("pred").unwrap(),
@@ -1092,36 +1092,44 @@ impl Context {
         .lhs
         .as_mut();
         match (
-            self.supertype_of(lhs.ref_t(), &target_t),
-            self.subtype_of(lhs.ref_t(), &target_t),
+            self.supertype_of(lhs.ref_t(), &cast_to),
+            self.subtype_of(lhs.ref_t(), &cast_to),
         ) {
             // assert 1 in {1}
-            (true, true) => {}
+            (true, true) => Ok(()),
             // assert x in Int (x: Nat)
-            (false, true) => {} // TODO: warn (needless)
+            (false, true) => Ok(()), // TODO: warn (needless)
             // assert x in Nat (x: Int)
             (true, false) => {
                 if let hir::Expr::Accessor(ref acc) = lhs {
-                    self.change_var_type(acc, target_t.clone())?;
+                    self.change_var_type(acc, cast_to.clone())?;
                 }
                 match lhs.ref_t() {
                     Type::FreeVar(fv) if fv.is_linked() => {
-                        let constraint = Constraint::new_subtype_of(target_t, Cyclicity::Not);
+                        let constraint = Constraint::new_subtype_of(cast_to, Cyclicity::Not);
                         fv.replace(FreeKind::new_unbound(self.level, constraint));
                     }
                     Type::FreeVar(fv) => {
-                        let new_constraint = Constraint::new_subtype_of(target_t, Cyclicity::Not);
+                        let new_constraint = Constraint::new_subtype_of(cast_to, Cyclicity::Not);
                         fv.update_constraint(new_constraint);
                     }
                     _ => {
-                        *lhs.ref_mut_t() = target_t;
+                        *lhs.ref_mut_t() = cast_to;
                     }
                 }
+                Ok(())
             }
             // assert x in Str (x: Int)
-            (false, false) => todo!(),
+            (false, false) => Err(TyCheckErrors::from(TyCheckError::invalid_type_cast_error(
+                self.cfg.input.clone(),
+                line!() as usize,
+                lhs.loc(),
+                self.caused_by(),
+                &lhs.to_string(),
+                &cast_to,
+                None,
+            ))),
         }
-        Ok(())
     }
 
     fn change_var_type(&mut self, acc: &hir::Accessor, t: Type) -> TyCheckResult<()> {
diff --git a/compiler/erg_compiler/error.rs b/compiler/erg_compiler/error.rs
index 023449ad..70cc0848 100644
--- a/compiler/erg_compiler/error.rs
+++ b/compiler/erg_compiler/error.rs
@@ -1568,6 +1568,34 @@ impl LowerError {
             caused_by,
         )
     }
+
+    #[allow(clippy::too_many_arguments)]
+    pub fn invalid_type_cast_error(
+        input: Input,
+        errno: usize,
+        loc: Location,
+        caused_by: AtomicStr,
+        name: &str,
+        cast_to: &Type,
+        hint: Option<AtomicStr>,
+    ) -> Self {
+        Self::new(
+            ErrorCore::new(
+                errno,
+                TypeError,
+                loc,
+                switch_lang!(
+                    "japanese" => format!("{YELLOW}{name}{RESET}の型を{RED}{cast_to}{RESET}にキャストすることはできません"),
+                    "simplified_chinese" => format!("{YELLOW}{name}{RESET}的类型无法转换为{RED}{cast_to}{RESET}"),
+                    "traditional_chinese" => format!("{YELLOW}{name}{RESET}的類型無法轉換為{RED}{cast_to}{RESET}"),
+                    "english" => format!("the type of {YELLOW}{name}{RESET} cannot be cast to {RED}{cast_to}{RESET}"),
+                ),
+                hint,
+            ),
+            input,
+            caused_by,
+        )
+    }
 }
 
 #[derive(Debug)]