diff --git a/redux/combine_reducers.py b/redux/combine_reducers.py
index 3d4286c..b6825ad 100644
--- a/redux/combine_reducers.py
+++ b/redux/combine_reducers.py
@@ -48,7 +48,7 @@ def combine_reducers(
         type[state_type],
         make_dataclass(
             state_type.__name__,
-            ('_id', *reducers.keys()),
+            {'_id', *reducers.keys(), *(field.name for field in fields(state_type))},
             frozen=True,
             kw_only=True,
         ),
@@ -66,28 +66,32 @@ def combined_reducer(
                 key = action.key
                 reducer = action.reducer
                 reducers[key] = reducer
+                field_names = {field.name for field in fields(state_class)}
                 state_class = make_dataclass(
                     state_type.__name__,
-                    ('_id', *reducers.keys()),
+                    {*field_names, key},
                     frozen=True,
                     kw_only=True,
                 )
-                reducer_result = reducer(
-                    None,
-                    CombineReducerInitAction(_id=_id, key=key),
+                reducer_result = (
+                    getattr(state, key)
+                    if hasattr(state, key)
+                    else reducer(
+                        None,
+                        CombineReducerInitAction(_id=_id, key=key),
+                    )
                 )
                 state = state_class(
-                    _id=state._id,  # noqa: SLF001
                     **(
                         {
-                            key_: (
+                            field: (
                                 reducer_result.state
                                 if is_complete_reducer_result(reducer_result)
                                 else reducer_result
                             )
-                            if key == key_
-                            else getattr(state, key_)
-                            for key_ in reducers
+                            if key == field
+                            else getattr(state, field)
+                            for field in field_names
                         }
                     ),
                 )
@@ -137,10 +141,15 @@ def combined_reducer(
             for key, reducer in reducers.items()
         }
         result_state = state_class(
-            _id=_id,
-            **{
-                key: result.state if is_complete_reducer_result(result) else result
-                for key, result in reducers_results.items()
+            **{  # pyright: ignore [reportArgumentType]
+                field.name: (
+                    reducers_results[field.name].state
+                    if is_complete_reducer_result(reducers_results[field.name])
+                    else reducers_results[field.name]
+                )
+                if field.name in reducers_results
+                else getattr(state, field.name, None)
+                for field in fields(state_class)
             },
         )
         result_actions += functools.reduce(