diff --git a/dargs/dargs.py b/dargs/dargs.py
index b9a1c9b..b590074 100644
--- a/dargs/dargs.py
+++ b/dargs/dargs.py
@@ -391,9 +391,14 @@ def _traverse_sub(
         variant_hook: HookVrntType = _DUMMYHOOK,
         path: list[str] | None = None,
     ):
-        assert isinstance(value, dict)
         if path is None:
             path = [self.name]
+        if not isinstance(value, dict):
+            raise ArgumentTypeError(
+                path,
+                f"key `{path[-1]}` gets wrong value type, "
+                f"requires dict but {type(value).__name__} is given",
+            )
         sub_hook(self, value, path)
         for subvrnt in self.sub_variants.values():
             variant_hook(subvrnt, value, path)
diff --git a/tests/test_checker.py b/tests/test_checker.py
index 92926be..f3800da 100644
--- a/tests/test_checker.py
+++ b/tests/test_checker.py
@@ -156,6 +156,14 @@ def test_sub_repeat_dict(self):
         }
         with self.assertRaises(ArgumentTypeError):
             ca.check(err_dict2)
+        err_dict3 = {
+            "base": {
+                "item1": {"sub1": 10, "sub2": "hello"},
+                "item2": "not_a_dict_error",
+            }
+        }
+        with self.assertRaises(ArgumentTypeError):
+            ca.check(err_dict3)
 
     def test_sub_variants(self):
         ca = Argument(