--- a/mercurial/util.py	Wed Mar 15 15:38:02 2017 -0700
+++ b/mercurial/util.py	Wed Mar 15 15:07:14 2017 -0700
@@ -1831,7 +1831,8 @@
 def strdate(string, format, defaults=None):
     """parse a localized time string and return a (unixtime, offset) tuple.
     if the string cannot be parsed, ValueError is raised."""
-    defaults = defaults or {}
+    if defaults is None:
+        defaults = {}
 
     # NOTE: unixtime = localunixtime + offset
     offset, date = parsetimezone(string)