diff --git a/tests/protocols/test_http.py b/tests/protocols/test_http.py
index 76fd9336c..677e72f9f 100644
--- a/tests/protocols/test_http.py
+++ b/tests/protocols/test_http.py
@@ -671,7 +671,19 @@ async def test_max_concurrency(http_protocol_cls: HTTPProtocol):
     protocol = get_connected_protocol(app, http_protocol_cls, limit_concurrency=1)
     protocol.data_received(SIMPLE_GET_REQUEST)
     await protocol.loop.run_one()
-    assert b"HTTP/1.1 503 Service Unavailable" in protocol.transport.buffer
+    assert (
+        b"\r\n".join(
+            [
+                b"HTTP/1.1 503 Service Unavailable",
+                b"content-type: text/plain; charset=utf-8",
+                b"content-length: 19",
+                b"connection: close",
+                b"",
+                b"Service Unavailable",
+            ]
+        )
+        == protocol.transport.buffer
+    )
 
 
 @pytest.mark.anyio
diff --git a/uvicorn/protocols/http/flow_control.py b/uvicorn/protocols/http/flow_control.py
index 893a26c80..42ecade71 100644
--- a/uvicorn/protocols/http/flow_control.py
+++ b/uvicorn/protocols/http/flow_control.py
@@ -1,12 +1,6 @@
 import asyncio
 
-from uvicorn._types import (
-    ASGIReceiveCallable,
-    ASGISendCallable,
-    HTTPResponseBodyEvent,
-    HTTPResponseStartEvent,
-    Scope,
-)
+from uvicorn._types import ASGIReceiveCallable, ASGISendCallable, Scope
 
 CLOSE_HEADER = (b"connection", b"close")
 
@@ -45,20 +39,16 @@ def resume_writing(self) -> None:
             self._is_writable_event.set()
 
 
-async def service_unavailable(scope: "Scope", receive: "ASGIReceiveCallable", send: "ASGISendCallable") -> None:
-    response_start: "HTTPResponseStartEvent" = {
-        "type": "http.response.start",
-        "status": 503,
-        "headers": [
-            (b"content-type", b"text/plain; charset=utf-8"),
-            (b"connection", b"close"),
-        ],
-    }
-    await send(response_start)
-
-    response_body: "HTTPResponseBodyEvent" = {
-        "type": "http.response.body",
-        "body": b"Service Unavailable",
-        "more_body": False,
-    }
-    await send(response_body)
+async def service_unavailable(scope: Scope, receive: ASGIReceiveCallable, send: ASGISendCallable) -> None:
+    await send(
+        {
+            "type": "http.response.start",
+            "status": 503,
+            "headers": [
+                (b"content-type", b"text/plain; charset=utf-8"),
+                (b"content-length", b"19"),
+                (b"connection", b"close"),
+            ],
+        }
+    )
+    await send({"type": "http.response.body", "body": b"Service Unavailable", "more_body": False})
diff --git a/uvicorn/protocols/http/httptools_impl.py b/uvicorn/protocols/http/httptools_impl.py
index 3fd5515af..60debaf8f 100644
--- a/uvicorn/protocols/http/httptools_impl.py
+++ b/uvicorn/protocols/http/httptools_impl.py
@@ -16,7 +16,6 @@
     ASGIReceiveEvent,
     ASGISendEvent,
     HTTPRequestEvent,
-    HTTPResponseBodyEvent,
     HTTPResponseStartEvent,
     HTTPScope,
 )
@@ -435,21 +434,18 @@ async def run_asgi(self, app: ASGI3Application) -> None:
             self.on_response = lambda: None
 
     async def send_500_response(self) -> None:
-        response_start_event: HTTPResponseStartEvent = {
-            "type": "http.response.start",
-            "status": 500,
-            "headers": [
-                (b"content-type", b"text/plain; charset=utf-8"),
-                (b"connection", b"close"),
-            ],
-        }
-        await self.send(response_start_event)
-        response_body_event: HTTPResponseBodyEvent = {
-            "type": "http.response.body",
-            "body": b"Internal Server Error",
-            "more_body": False,
-        }
-        await self.send(response_body_event)
+        await self.send(
+            {
+                "type": "http.response.start",
+                "status": 500,
+                "headers": [
+                    (b"content-type", b"text/plain; charset=utf-8"),
+                    (b"content-length", b"21"),
+                    (b"connection", b"close"),
+                ],
+            }
+        )
+        await self.send({"type": "http.response.body", "body": b"Internal Server Error", "more_body": False})
 
     # ASGI interface
     async def send(self, message: ASGISendEvent) -> None: