diff --git a/mkdocs/config/base.py b/mkdocs/config/base.py
index c3b01e63..9a6099fa 100644
--- a/mkdocs/config/base.py
+++ b/mkdocs/config/base.py
@@ -17,6 +17,18 @@ class BaseConfigOption:
         self.warnings = []
         self.default = None
 
+    @property
+    def default(self):
+        try:
+            # ensure no mutable values are assigned
+            return self._default.copy()
+        except AttributeError:
+            return self._default
+
+    @default.setter
+    def default(self, value):
+        self._default = value
+
     def validate(self, value):
         return self.run_validation(value)
 
diff --git a/mkdocs/config/config_options.py b/mkdocs/config/config_options.py
index 25499ac1..887ff693 100644
--- a/mkdocs/config/config_options.py
+++ b/mkdocs/config/config_options.py
@@ -69,13 +69,9 @@ class OptionallyRequired(BaseConfigOption):
 
         if value is None:
             if self.default is not None:
-                if hasattr(self.default, 'copy'):
-                    # ensure no mutable values are assigned
-                    value = self.default.copy()
-                else:
-                    value = self.default
+                value = self.default
             elif not self.required:
-                return
+                return None
             elif self.required:
                 raise ValidationError("Required configuration not provided.")