diff --git a/mercurial/templater.py b/mercurial/templater.py
--- a/mercurial/templater.py
+++ b/mercurial/templater.py
@@ -1326,7 +1326,7 @@
         # We can only hit this on Python 3, and it's here to guard
         # against infinite recursion.
         raise error.ProgrammingError('Mercurial IO including templates is done'
-                                     ' with bytes, not strings')
+                                     ' with bytes, not strings, got %r' % thing)
     elif thing is None:
         pass
     elif not util.safehasattr(thing, '__iter__'):