diff --git a/jupyterlab_server/config.py b/jupyterlab_server/config.py
index 51ec2b40..50ce42b1 100644
--- a/jupyterlab_server/config.py
+++ b/jupyterlab_server/config.py
@@ -96,8 +96,8 @@ def get_page_config(labextensions_path, app_settings_dir=None, logger=None):
 
             # Convert lists to dicts
             for key in [disabled_key, "deferredExtensions"]:
-                if key in data:
-                    data[key] = dict((key, True) for key in data[key])
+                if key in data and not isinstance(data[key], dict):
+                    data[key] = dict((k, True) for k in data[key])
 
             recursive_update(page_config, data)