diff -r affcecf20c15 -r 73d0a3dd7e53 mercurial/utils/stringutil.py
--- a/mercurial/utils/stringutil.py	Fri Apr 13 23:50:17 2018 -0400
+++ b/mercurial/utils/stringutil.py	Fri Apr 13 23:55:37 2018 -0400
@@ -42,6 +42,8 @@
         return '%d' % o
     elif isinstance(o, float):
         return '%f' % o
+    elif o is None:
+        return b'None'
     else:
         raise error.ProgrammingError('do not know how to format %r' % o)