diff --git a/draccus/wrappers/choice_wrapper.py b/draccus/wrappers/choice_wrapper.py
index a52a2f7..ec7fe8e 100644
--- a/draccus/wrappers/choice_wrapper.py
+++ b/draccus/wrappers/choice_wrapper.py
@@ -3,7 +3,7 @@
 import inspect
 from dataclasses import Field
 from functools import cached_property
-from typing import Dict, Optional, Type
+from typing import Dict, Optional, Sequence, Type
 
 from ..choice_types import CHOICE_TYPE_KEY, ChoiceType
 from ..parsers.decoding import has_custom_decoder
@@ -195,7 +195,7 @@ def register_actions(self, parser: argparse.ArgumentParser) -> None:
 
         has_field_wrapper = False
 
-        for child in children.values():
+        for child in children:
             from .dataclass_wrapper import DataclassWrapper
 
             if isinstance(child, DataclassWrapper):
@@ -225,7 +225,7 @@ def register_actions(self, parser: argparse.ArgumentParser) -> None:
             )
 
     @cached_property
-    def _children(self) -> Dict[str, Optional[Wrapper]]:
+    def _children(self) -> Sequence[Optional[Wrapper]]:
         from .dataclass_wrapper import DataclassWrapper
         from .field_wrapper import FieldWrapper
 
@@ -246,6 +246,9 @@ def _wrap_child(child: Type) -> Optional[Wrapper]:
             elif child is None or child is type(None):
                 return None
             else:
-                raise ValueError(f"Unexpected child type: {child}")
+                assert self._field is not None
+                wrapper = FieldWrapper(parent=self.parent, field=self._field, preferred_help=self.preferred_help)
+                wrapper.required = False
+                return wrapper
 
-        return {child.__name__: _wrap_child(child) for child in self.union.__args__}
+        return [_wrap_child(child) for child in self.union.__args__]
diff --git a/draccus/wrappers/dataclass_wrapper.py b/draccus/wrappers/dataclass_wrapper.py
index faf8ac7..2b397f6 100644
--- a/draccus/wrappers/dataclass_wrapper.py
+++ b/draccus/wrappers/dataclass_wrapper.py
@@ -180,7 +180,6 @@ def _wrap_field(
         return child_wrapper
 
     elif utils.is_optional_or_union_with_dataclass_type_arg(field.type):
-        # TODO(dlwh): I don't like this. Add UnionWrapper or something
         name = field.name
         from .choice_wrapper import UnionWrapper
 
diff --git a/tests/test_union.py b/tests/test_union.py
index 354d3a5..3a96e30 100644
--- a/tests/test_union.py
+++ b/tests/test_union.py
@@ -1,5 +1,5 @@
-from dataclasses import dataclass
-from typing import Union
+from dataclasses import dataclass, field
+from typing import Dict, Union
 
 import pytest
 
@@ -148,3 +148,16 @@ class Foo(TestSetup):
     Bar: The fields `y` are not valid for Bar""".strip() in str(
         e.value
     )
+
+
+def test_union_argparse_dict():
+    @dataclass
+    class Bar:
+        y: int
+
+    @dataclass
+    class Foo(TestSetup):
+        x: Optional[Union[Bar, Dict[str, Bar]]] = field(default=None)
+
+    foo = Foo.setup('--x \'{"a": {"y": 1}, "b": {"y": 2}}\'')
+    assert foo.x == {"a": Bar(y=1), "b": Bar(y=2)}