diff --git a/fastapi/dependencies/utils.py b/fastapi/dependencies/utils.py
index d43fa8a516..0f25a3c360 100644
--- a/fastapi/dependencies/utils.py
+++ b/fastapi/dependencies/utils.py
@@ -902,8 +902,9 @@ async def _extract_form_body(
             value = serialize_sequence_value(field=field, value=results)
         if value is not None:
             values[field.alias] = value
+    field_aliases = {field.alias for field in body_fields}
     for key, value in received_body.items():
-        if key not in values:
+        if key not in field_aliases:
             values[key] = value
     return values
 
diff --git a/tests/test_form_default.py b/tests/test_form_default.py
new file mode 100644
index 0000000000..2a12049d1a
--- /dev/null
+++ b/tests/test_form_default.py
@@ -0,0 +1,35 @@
+from typing import Optional
+
+from fastapi import FastAPI, File, Form
+from starlette.testclient import TestClient
+from typing_extensions import Annotated
+
+app = FastAPI()
+
+
+@app.post("/urlencoded")
+async def post_url_encoded(age: Annotated[Optional[int], Form()] = None):
+    return age
+
+
+@app.post("/multipart")
+async def post_multi_part(
+    age: Annotated[Optional[int], Form()] = None,
+    file: Annotated[Optional[bytes], File()] = None,
+):
+    return {"file": file, "age": age}
+
+
+client = TestClient(app)
+
+
+def test_form_default_url_encoded():
+    response = client.post("/urlencoded", data={"age": ""})
+    assert response.status_code == 200
+    assert response.text == "null"
+
+
+def test_form_default_multi_part():
+    response = client.post("/multipart", data={"age": ""})
+    assert response.status_code == 200
+    assert response.json() == {"file": None, "age": None}