diff --git a/mercurial/templater.py b/mercurial/templater.py
--- a/mercurial/templater.py
+++ b/mercurial/templater.py
@@ -427,7 +427,7 @@
         sym = findsymbolicname(arg)
         if sym:
             msg = (_("template filter '%s' is not compatible with keyword '%s'")
-                   % (filt.__name__, sym))
+                   % (filt.__name__.encode('ascii'), sym))
         else:
             msg = _("incompatible use of template filter '%s'") % filt.__name__
         raise error.Abort(msg)