diff --git a/gunicorn/config.py b/gunicorn/config.py
index 64c12d077..75e4c7cf1 100644
--- a/gunicorn/config.py
+++ b/gunicorn/config.py
@@ -281,6 +281,9 @@ def __lt__(self, other):
 
 
 def validate_bool(val):
+    if val is None:
+        return
+
     if isinstance(val, bool):
         return val
     if not isinstance(val, six.string_types):