diff --git a/fastapi/_compat/v2.py b/fastapi/_compat/v2.py
index c913874612..2dabdc69c1 100644
--- a/fastapi/_compat/v2.py
+++ b/fastapi/_compat/v2.py
@@ -585,10 +585,10 @@ if shared.PYDANTIC_VERSION_MINOR_TUPLE >= (2, 6):
     # Omit by default for scalar mapping and scalar sequence mapping annotations
     # added in Pydantic v2.6 https://github.com/pydantic/pydantic/releases/tag/v2.6.0
     def _omit_by_default(annotation: Any) -> Any:
-        origin = getattr(annotation, "__origin__", None)
-        args = getattr(annotation, "__args__", ())
+        origin = get_origin(annotation)
+        args = get_args(annotation)
 
-        if origin is Union:
+        if origin is Union or origin is UnionType:
             new_args = tuple(_omit_by_default(arg) for arg in args)
             return Union[new_args]
         elif origin is list:
diff --git a/tests/test_request_params/test_query/test_free_form.py b/tests/test_request_params/test_query/test_free_form.py
index d2f28bd952..a0fd28aaa5 100644
--- a/tests/test_request_params/test_query/test_free_form.py
+++ b/tests/test_request_params/test_query/test_free_form.py
@@ -1,4 +1,4 @@
-from typing import Annotated
+from typing import Annotated, Union
 
 import pytest
 from dirty_equals import IsOneOf
@@ -82,7 +82,7 @@ def test_required_dict_str(path: str):
 
 @app.get("/required-dict-union")
 async def read_required_dict_union(
-    p: Annotated[dict[str, str] | dict[str, int], Query()],
+    p: Annotated[Union[dict[str, str], dict[str, int]], Query()],
 ):
     return {"p": p}