diff --git a/llsd_asgi/middleware.py b/llsd_asgi/middleware.py
index 4dc05bd..2d35841 100644
--- a/llsd_asgi/middleware.py
+++ b/llsd_asgi/middleware.py
@@ -147,8 +147,8 @@ async def send_with_llsd(self, message: Message) -> None:
             if self.accept_header:
                 headers["Content-Type"] = self.accept_header
             else:
-                # quirks mode allows a response without a Content-Type response header
-                del headers["Content-Type"]
+                # quirks mode allows a response to default to application/xml instead of application/llsd+xml
+                headers["Content-Type"] = "application/xml"
             headers["Content-Length"] = str(len(body))
             message["body"] = body
 
diff --git a/tests/test_middleware.py b/tests/test_middleware.py
index 1f4662b..a7e2d66 100644
--- a/tests/test_middleware.py
+++ b/tests/test_middleware.py
@@ -196,7 +196,7 @@ async def test_quirks_encode(accept: str) -> None:
             headers["accept"] = accept
         r = await client.get("/")
         assert r.status_code == 200
-        assert "content-type" not in r.headers
+        assert r.headers["content-type"] == "application/xml"
         assert llsd.parse_xml(r.content) == {"message": "Hello, world!"}