diff --git a/dargs/dargs.py b/dargs/dargs.py
index dc70b0d..6a7c87c 100644
--- a/dargs/dargs.py
+++ b/dargs/dargs.py
@@ -224,7 +224,11 @@ def _reorg_dtype(self):
         # check conner cases
         if self.sub_fields or self.sub_variants:
             self.dtype.add(list if self.repeat else dict)
-        if self.optional and self.default is not _Flags.NONE:
+        if (
+            self.optional
+            and self.default is not _Flags.NONE
+            and all([not isinstance_annotation(self.default, tt) for tt in self.dtype])
+        ):
             self.dtype.add(type(self.default))
         # and make it compatible with `isinstance`
         self.dtype = tuple(self.dtype)
@@ -968,6 +972,19 @@ def trim_by_pattern(
         argdict.pop(key)
 
 
+def isinstance_annotation(value, dtype) -> bool:
+    """Same as isinstance(), but supports arbitrary type annotations."""
+    try:
+        typeguard.check_type(
+            value,
+            dtype,
+            collection_check_strategy=typeguard.CollectionCheckStrategy.ALL_ITEMS,
+        )
+    except typeguard.TypeCheckError as e:
+        return False
+    return True
+
+
 class ArgumentEncoder(json.JSONEncoder):
     """Extended JSON Encoder to encode Argument object:
 
diff --git a/dargs/sphinx.py b/dargs/sphinx.py
index 80a80e3..5cc9e7f 100644
--- a/dargs/sphinx.py
+++ b/dargs/sphinx.py
@@ -192,5 +192,11 @@ def _test_arguments() -> List[Argument]:
     return [
         Argument(name="test1", dtype=int, doc="Argument 1"),
         Argument(name="test2", dtype=[float, None], doc="Argument 2"),
-        Argument(name="test3", dtype=List[str], doc="Argument 3"),
+        Argument(
+            name="test3",
+            dtype=List[str],
+            default=["test"],
+            optional=True,
+            doc="Argument 3",
+        ),
     ]
diff --git a/tests/test_checker.py b/tests/test_checker.py
index 088a8b0..62be7ec 100644
--- a/tests/test_checker.py
+++ b/tests/test_checker.py
@@ -31,6 +31,9 @@ def test_name_type(self):
         # list[int]
         ca = Argument("key1", List[float])
         ca.check({"key1": [1, 2.0, 3]})
+        with self.assertRaises(ArgumentTypeError):
+            ca.check({"key1": [1, 2.0, "3"]})
+        ca = Argument("key1", List[float], default=[], optional=True)
         with self.assertRaises(ArgumentTypeError):
             ca.check({"key1": [1, 2.0, "3"]})
         # optional case