--- a/mercurial/formatter.py	Mon Jul 20 21:32:10 2020 -0700
+++ b/mercurial/formatter.py	Mon Jul 20 21:33:28 2020 -0700
@@ -551,6 +551,8 @@
 
 
 def literal_templatespec(tmpl):
+    if pycompat.ispy3:
+        assert not isinstance(tmpl, str), b'tmpl must not be a str'
     return templatespec(b'', tmpl, None)