diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -45,7 +45,7 @@
                      if not c.isalnum())
 
 # The config knobs that will be altered (if unset) by ui.tweakdefaults.
-tweakrc = """
+tweakrc = b"""
 [ui]
 # The rollback command is dangerous. As a rule, don't use it.
 rollback = False