diff --git a/tests/test_custom_openapi_subapp.py b/tests/test_custom_openapi_subapp.py
new file mode 100644
index 000000000..50abcd3fa
--- /dev/null
+++ b/tests/test_custom_openapi_subapp.py
@@ -0,0 +1,39 @@
+from fastapi import FastAPI
+from fastapi.openapi.utils import get_openapi
+from fastapi.testclient import TestClient
+
+swagger_ui_oauth2_redirect_url = "/docs/redirect"
+
+app = FastAPI()
+sub_app = FastAPI(
+    docs_url=None,
+    redoc_url=None,
+    openapi_url=None,
+    )
+
+@sub_app.get("/items/")
+async def read_items():
+    return {"id": "foo"}
+
+@sub_app.get("/openapi.json")
+async def openapi():
+    return get_openapi(
+        title="Custom OpenAPI",
+        version="0.1",
+        routes=sub_app.routes,
+        prefix="/sub_app",
+    )
+
+app.mount("/sub_app", sub_app)
+
+
+client = TestClient(app)
+
+
+def test_sub_app_open_api():
+    response = client.get("/sub_app/openapi.json")
+    assert response.status_code == 200, response.json()
+    assert response.headers["content-type"] == "application/json"
+    paths = list(response.json()["paths"].keys())
+    assert paths == [
+            "/sub_app/items/", "/sub_app/openapi.json",]