diff --git a/mercurial/pure/base85.py b/mercurial/pure/base85.py
--- a/mercurial/pure/base85.py
+++ b/mercurial/pure/base85.py
@@ -67,7 +67,7 @@ def b85decode(text):
                 acc = acc * 85 + _b85dec[c]
             except KeyError:
                 raise ValueError(
-                    b'bad base85 character at position %d' % (i + j)
+                    'bad base85 character at position %d' % (i + j)
                 )
         if acc > 4294967295:
             raise ValueError(b'Base85 overflow in hunk starting at byte %d' % i)