diff --git a/fastapi/dependencies/utils.py b/fastapi/dependencies/utils.py
index 84dfa4d03..603dfa231 100644
--- a/fastapi/dependencies/utils.py
+++ b/fastapi/dependencies/utils.py
@@ -845,6 +845,7 @@ async def _extract_form_body(
     values = {}
     first_field = body_fields[0]
     first_field_info = first_field.field_info
+    processed_keys = set()
 
     for field in body_fields:
         value = _get_multidict_value(field, received_body)
@@ -873,10 +874,11 @@ async def _extract_form_body(
                 for sub_value in value:
                     tg.start_soon(process_fn, sub_value.read)
             value = serialize_sequence_value(field=field, value=results)
+        processed_keys.add(field.alias)
         if value is not None:
             values[field.alias] = value
     for key, value in received_body.items():
-        if key not in values:
+        if key not in processed_keys:
             values[key] = value
     return values
 
diff --git a/tests/test_form_default.py b/tests/test_form_default.py
new file mode 100644
index 000000000..97ff47463
--- /dev/null
+++ b/tests/test_form_default.py
@@ -0,0 +1,47 @@
+from typing import Optional
+
+from fastapi import FastAPI, File, Form
+from starlette.testclient import TestClient
+from typing_extensions import Annotated
+
+app = FastAPI()
+client = TestClient(app)
+
+
+@app.post("/urlencoded")
+async def post_url_encoded(age: Annotated[Optional[int], Form()] = None):
+    return age
+
+
+def test_form_default_url_encoded():
+    response = client.post("/urlencoded", data={"age": ""})
+    assert response.status_code == 200
+    assert response.text == "null"
+
+
+@app.post("/multipart")
+async def post_multi_part(
+    age: Annotated[Optional[int], Form()] = None,
+    file: Annotated[Optional[bytes], File()] = None,
+):
+    assert file is None
+    assert age is None
+
+
+def test_form_default_multi_part():
+    response = client.post("/multipart", data={"age": ""})
+    assert response.status_code == 200
+
+
+@app.post("/multipart-file-first")
+async def post_multi_part_file_first(
+    file: Annotated[Optional[bytes], File()] = None,
+    age: Annotated[Optional[int], Form()] = None,
+):
+    assert file is None
+    assert age is None
+
+
+def test_form_default_multi_part_file_first():
+    response = client.post("/multipart-file-first", data={"age": ""})
+    assert response.status_code == 200