diff --git a/starlette/responses.py b/starlette/responses.py
index a6ebe8c..4390c29 100644
--- a/starlette/responses.py
+++ b/starlette/responses.py
@@ -31,13 +31,16 @@ class Response:
 
     def __init__(
         self,
-        content: typing.Any,
+        content: typing.Any = None,
         status_code: int = 200,
         headers: dict = None,
         media_type: str = None,
         background: BackgroundTask = None,
     ) -> None:
-        self.body = self.render(content)
+        if content is None:
+            self.body = b""
+        else:
+            self.body = self.render(content)
         self.status_code = status_code
         if media_type is not None:
             self.media_type = media_type
@@ -63,8 +66,8 @@ class Response:
             populate_content_length = b"content-length" in keys
             populate_content_type = b"content-type" in keys
 
-        body = getattr(self, "body", None)
-        if body is not None and populate_content_length:
+        body = getattr(self, "body", b"")
+        if body and populate_content_length:
             content_length = str(len(body))
             raw_headers.append((b"content-length", content_length.encode("latin-1")))
 

diff --git a/tests/test_responses.py b/tests/test_responses.py
index 7e23e29..e68958e 100644
--- a/tests/test_responses.py
+++ b/tests/test_responses.py
@@ -126,11 +126,11 @@ def test_response_headers():
 
 def test_response_phrase():
     def app(scope):
-        return Response(b"", status_code=200)
+        return Response(status_code=204)
 
     client = TestClient(app)
     response = client.get("/")
-    assert response.reason == "OK"
+    assert response.reason == "No Content"
 
     def app(scope):
         return Response(b"", status_code=123)

the raw "Response" should'nt mandatory a content ... no ?
I need to manage myself the etag/304 response ...

to send a 304 if-none-match, I use : 

    Response(status_code=304,content="",headers=rheaders)

But the body content should not be written ... here the content is mandatory : None cause troubble, and empty string is an empty string ...


tests/test_responses.py::test_response_phrase
tests/test_responses.py::test_text_response tests/test_responses.py::test_bytes_response tests/test_responses.py::test_ujson_response tests/test_responses.py::test_redirect_response tests/test_responses.py::test_streaming_response tests/test_responses.py::test_response_headers tests/test_responses.py::test_file_response tests/test_responses.py::test_file_response_with_directory_raises_error tests/test_responses.py::test_file_response_with_missing_file_raises_error tests/test_responses.py::test_set_cookie tests/test_responses.py::test_delete_cookie