diff --git a/gunicorn/config.py b/gunicorn/config.py
index 64c12d07..75e4c7cf 100644
--- a/gunicorn/config.py
+++ b/gunicorn/config.py
@@ -281,6 +281,9 @@ Setting = SettingMeta('Setting', (Setting,), {})
 
 
 def validate_bool(val):
+    if val is None:
+        return
+
     if isinstance(val, bool):
         return val
     if not isinstance(val, six.string_types):