diff --git a/examples/service-fastapi/jupyterhub_config.py b/examples/service-fastapi/jupyterhub_config.py
index 2b0e830b..9fb054ec 100644
--- a/examples/service-fastapi/jupyterhub_config.py
+++ b/examples/service-fastapi/jupyterhub_config.py
@@ -1,16 +1,31 @@
 import os
+import warnings
 
-service = {
-    "name": "fastapi",
-    "url": "http://127.0.0.1:10202",
-    "command": ["uvicorn", "app:app", "--port", "10202"],
-}
-# If running behind a proxy, or in Docker / Kubernetes infrastructure,
-# you probably need to set a different public Hub host than the
-# internal JUPYTERHUB_API_URL host
-if "PUBLIC_HOST" in os.environ:
-    public_host = os.environ["PUBLIC_HOST"]
-    service["oauth_redirect_uri"] = f"{public_host}/services/fastapi/oauth_callback"
-    service["environment"] = {"PUBLIC_HOST": public_host}
+# When Swagger performs OAuth2 in the browser, it will set
+# the request host + relative path as the redirect uri, causing a
+# uri mismatch if the oauth_redirect_uri is just the relative path
+# is set in the c.JupyterHub.services entry (as per default).
+# Therefore need to know the request host ahead of time.
+if "PUBLIC_HOST" not in os.environ:
+    msg = (
+        "env PUBLIC_HOST is not set, defaulting to http://127.0.0.1:8000.  "
+        "This can cause problems with OAuth.  "
+        "Set PUBLIC_HOST to your public (browser accessible) host."
+    )
+    warnings.warn(msg)
+    public_host = "http://127.0.0.1:8000"
+else:
+    public_host = os.environ["PUBLIC_HOST"].rstrip('/')
+service_name = "fastapi"
+oauth_redirect_uri = f"{public_host}/services/{service_name}/oauth_callback"
 
-c.JupyterHub.services = [service]
+c.JupyterHub.services = [
+    {
+        "name": service_name,
+        "url": "http://127.0.0.1:10202",
+        "command": ["uvicorn", "app:app", "--port", "10202"],
+        "admin": True,
+        "oauth_redirect_uri": oauth_redirect_uri,
+        "environment": {"PUBLIC_HOST": public_host},
+    }
+]