diff --git a/onnxslim/argparser.py b/onnxslim/argparser.py
index 769b909..bddc25e 100644
--- a/onnxslim/argparser.py
+++ b/onnxslim/argparser.py
@@ -7,6 +7,12 @@
 
 import onnxslim
 
+from typing import Union, List, Optional, get_origin, get_args
+
+def _get_inner_type(arg_type):
+    if get_origin(arg_type) is Union:
+        return next((t for t in get_args(arg_type) if t is not type(None)), str)
+    return arg_type
 
 @dataclass
 class ModelArguments:
@@ -126,12 +132,17 @@ def __init__(self, *argument_dataclasses: Type, **kwargs):
     def _add_arguments(self):
         for dataclass_type in self.argument_dataclasses:
             for field_name, field_def in dataclass_type.__dataclass_fields__.items():
-                arg_type = field_def.type
+                arg_type = _get_inner_type(field_def.type)
                 default_value = field_def.default if field_def.default is not field_def.default_factory else None
                 help_text = field_def.metadata.get("help", "")
-                nargs = "+" if arg_type == Optional[List[str]] else None
+                nargs = "+" if arg_type == list else None
                 choices = field_def.metadata.get("choices", None)
 
+                if choices and default_value is not None and default_value not in choices:
+                    raise ValueError(
+                        f"Invalid default value '{default_value}' for argument '{field_name}'. Must be one of {choices}."
+                    )
+
                 if arg_type == bool:
                     self.parser.add_argument(
                         f"--{field_name.replace('_', '-')}",
@@ -142,7 +153,7 @@ def _add_arguments(self):
                 else:
                     self.parser.add_argument(
                         f"--{field_name.replace('_', '-')}",
-                        type=arg_type if arg_type != Optional[List[str]] else str,
+                        type=arg_type,
                         default=default_value,
                         nargs=nargs,
                         choices=choices,