--- a/mercurial/templateutil.py	Fri Jun 08 20:57:54 2018 +0900
+++ b/mercurial/templateutil.py	Tue Jun 05 21:40:33 2018 +0900
@@ -155,6 +155,8 @@
         raise error.ParseError(_('%r is not iterable') % self._value)
 
     def show(self, context, mapping):
+        if self._value is None:
+            return b''
         return pycompat.bytestr(self._value)
 
     def tovalue(self, context, mapping):