diff --git a/fastapi/responses.py b/fastapi/responses.py
index c915cc7cb..34fc72986 100644
--- a/fastapi/responses.py
+++ b/fastapi/responses.py
@@ -21,9 +21,9 @@ except ImportError:  # pragma: nocover
 
 
 try:
-    import bson
+    import bson  # type: ignore[import-untyped]
 except ImportError:  # pragma: nocover
-    bson = None  # type: ignore
+    bson = None
 
 
 class UJSONResponse(JSONResponse):
diff --git a/tests/test_bson_response_class.py b/tests/test_bson_response_class.py
new file mode 100644
index 000000000..b603c3366
--- /dev/null
+++ b/tests/test_bson_response_class.py
@@ -0,0 +1,21 @@
+import bson
+
+from fastapi import FastAPI
+from fastapi.responses import BSONResponse
+from fastapi.testclient import TestClient
+
+app = FastAPI(default_response_class=BSONResponse)
+
+
+@app.get("/bson_keys")
+def get_bson_serialized_data():
+    return {"key": "Hello World", 1: 1}
+
+
+client = TestClient(app)
+
+
+def test_bson_serialized_data():
+    with client:
+        response = client.get("/bson_keys")
+    assert response.content == bson.dumps({"key": "Hello World", 1: 1})