diff --git a/jupyterlab_server/handlers.py b/jupyterlab_server/handlers.py
index 1aa241a..aeff694 100644
--- a/jupyterlab_server/handlers.py
+++ b/jupyterlab_server/handlers.py
@@ -179,7 +179,8 @@ class NotFoundHandler(LabHandler):
     @lru_cache  # noqa: B019
     def get_page_config(self) -> dict[str, Any]:
         """Get the page config."""
-        page_config = super().get_page_config()
+        # Making a copy of the page_config to ensure changes do not affect the original
+        page_config = super().get_page_config().copy()
         page_config["notFoundUrl"] = self.request.path
         return page_config