diff --git a/datasette/app.py b/datasette/app.py
index f43ec205..6a7a6c6d 100644
--- a/datasette/app.py
+++ b/datasette/app.py
@@ -213,7 +213,7 @@ class Datasette:
             and not inspect_data
         ):
             inspect_data = json.loads((config_dir / "inspect-data.json").read_text())
-            if immutables is None:
+            if not immutables:
                 immutable_filenames = [i["file"] for i in inspect_data.values()]
                 immutables = [
                     f for f in self.files if Path(f).name in immutable_filenames
diff --git a/docs/settings.rst b/docs/settings.rst
index f2467aa4..b4c8a50e 100644
--- a/docs/settings.rst
+++ b/docs/settings.rst
@@ -48,7 +48,7 @@ The files that can be included in this directory are as follows. All are optiona
 
 * ``*.db`` - SQLite database files that will be served by Datasette
 * ``metadata.json`` - :ref:`metadata` for those databases - ``metadata.yaml`` or ``metadata.yml`` can be used as well
-* ``inspect-data.json`` - the result of running ``datasette inspect`` - any database files listed here will be treated as immutable, so they should not be changed while Datasette is running
+* ``inspect-data.json`` - the result of running ``datasette inspect *.db --inspect-file=inspect-data.json`` from the configuration directory - any database files listed here will be treated as immutable, so they should not be changed while Datasette is running
 * ``settings.json`` - settings that would normally be passed using ``--setting`` - here they should be stored as a JSON object of key/value pairs
 * ``templates/`` - a directory containing :ref:`customization_custom_templates`
 * ``plugins/`` - a directory containing plugins, see :ref:`writing_plugins_one_off`