diff --git a/libs/core/langchain_core/runnables/passthrough.py b/libs/core/langchain_core/runnables/passthrough.py
index 63933394d2b..2dceff9ca30 100644
--- a/libs/core/langchain_core/runnables/passthrough.py
+++ b/libs/core/langchain_core/runnables/passthrough.py
@@ -434,14 +434,17 @@ class RunnableAssign(RunnableSerializable[Dict[str, Any], Dict[str, Any]]):
         if not issubclass(map_input_schema, RootModel) and not issubclass(
             map_output_schema, RootModel
         ):
-            # ie. both are dicts
+            fields = {}
+
+            for name, field_info in map_input_schema.model_fields.items():
+                fields[name] = (field_info.annotation, field_info.default)
+
+            for name, field_info in map_output_schema.model_fields.items():
+                fields[name] = (field_info.annotation, field_info.default)
+
             return create_model(  # type: ignore[call-overload]
                 "RunnableAssignOutput",
-                **{
-                    k: (v.type_, v.default)
-                    for s in (map_input_schema, map_output_schema)
-                    for k, v in s.model_fields.items()
-                },
+                **fields,
             )
         elif not issubclass(map_output_schema, RootModel):
             # ie. only map output is a dict