diff --git a/mkdocs/config/config_options.py b/mkdocs/config/config_options.py
index 9aa498ec..3b4fa9ad 100644
--- a/mkdocs/config/config_options.py
+++ b/mkdocs/config/config_options.py
@@ -139,7 +139,7 @@ class ListOfItems(Generic[T], BaseConfigOption[List[T]]):
 
     required: Union[bool, None] = None  # Only for subclasses to set.
 
-    def __init__(self, option_type: BaseConfigOption[T], default: List[T] = []):
+    def __init__(self, option_type: BaseConfigOption[T], default=None):
         super().__init__()
         self.default = default
         self.option_type = option_type
@@ -153,11 +153,14 @@ class ListOfItems(Generic[T], BaseConfigOption[List[T]]):
         self._key_name = key_name
 
     def run_validation(self, value):
-        if value is None and not self.required:
-            return self.default
-
+        if value is None:
+            if self.required or self.default is None:
+                raise ValidationError("Required configuration not provided.")
+            value = self.default
         if not isinstance(value, list):
             raise ValidationError(f'Expected a list of items, but a {type(value)} was given.')
+        if not value:  # Optimization for empty list
+            return value
 
         fake_config = Config(())
         try:
@@ -198,7 +201,7 @@ class ConfigItems(ListOfItems[LegacyConfig]):
         ...
 
     def __init__(self, *config_options: PlainConfigSchemaItem, required=None):
-        super().__init__(SubConfig(*config_options))
+        super().__init__(SubConfig(*config_options), default=[])
         self._legacy_required = required
         self.required = bool(required)
 
diff --git a/mkdocs/tests/config/config_options_tests.py b/mkdocs/tests/config/config_options_tests.py
index 0f795d4c..221ddb60 100644
--- a/mkdocs/tests/config/config_options_tests.py
+++ b/mkdocs/tests/config/config_options_tests.py
@@ -606,7 +606,7 @@ class ListOfItemsTest(TestCase):
 
     def test_list_default(self):
         class Schema:
-            option = c.ListOfItems(c.Type(int))
+            option = c.ListOfItems(c.Type(int), default=[])
 
         conf = self.get_config(Schema, {})
         self.assertEqual(conf['option'], [])
@@ -614,15 +614,15 @@ class ListOfItemsTest(TestCase):
         conf = self.get_config(Schema, {'option': None})
         self.assertEqual(conf['option'], [])
 
-    def test_none_default(self):
+    def test_none_without_default(self):
         class Schema:
-            option = c.ListOfItems(c.Type(str), default=None)
+            option = c.ListOfItems(c.Type(str))
 
-        conf = self.get_config(Schema, {})
-        self.assertEqual(conf['option'], None)
+        with self.expect_error(option="Required configuration not provided."):
+            conf = self.get_config(Schema, {})
 
-        conf = self.get_config(Schema, {'option': None})
-        self.assertEqual(conf['option'], None)
+        with self.expect_error(option="Required configuration not provided."):
+            conf = self.get_config(Schema, {'option': None})
 
         conf = self.get_config(Schema, {'option': ['foo']})
         self.assertEqual(conf['option'], ['foo'])
@@ -1279,7 +1279,8 @@ class ConfigItemsTest(TestCase):
                 c.SubConfig(
                     ('opt', c.Type(int)),
                     validate=True,
-                )
+                ),
+                default=[],
             )
 
         conf = self.get_config(Schema, {})
@@ -1305,11 +1306,11 @@ class ConfigItemsTest(TestCase):
                 )
             )
 
-        conf = self.get_config(Schema, {})
-        self.assertEqual(conf['sub'], [])
+        with self.expect_error(sub="Required configuration not provided."):
+            conf = self.get_config(Schema, {})
 
-        conf = self.get_config(Schema, {'sub': None})
-        self.assertEqual(conf['sub'], [])
+        with self.expect_error(sub="Required configuration not provided."):
+            conf = self.get_config(Schema, {'sub': None})
 
         with self.expect_error(
             sub="Sub-option 'opt' configuration error: Expected type: <class 'int'> but received: <class 'str'>"