--- a/mercurial/util.py	Sun Mar 12 21:53:03 2017 -0700
+++ b/mercurial/util.py	Sun Mar 12 21:54:32 2017 -0700
@@ -1827,9 +1827,11 @@
 
     return None, s
 
-def strdate(string, format, defaults=[]):
+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 []
+
     # NOTE: unixtime = localunixtime + offset
     offset, date = parsetimezone(string)