--- a/mercurial/hgweb/common.py	Wed Mar 15 15:10:09 2017 -0700
+++ b/mercurial/hgweb/common.py	Wed Mar 15 15:11:04 2017 -0700
@@ -96,7 +96,9 @@
             message = _statusmessage(code)
         Exception.__init__(self, message)
         self.code = code
-        self.headers = headers or []
+        if headers is None:
+            headers = []
+        self.headers = headers
 
 class continuereader(object):
     def __init__(self, f, write):