diff --git a/hgext/fastannotate/revmap.py b/hgext/fastannotate/revmap.py
--- a/hgext/fastannotate/revmap.py
+++ b/hgext/fastannotate/revmap.py
@@ -207,7 +207,7 @@
             path = self.rev2path(rev)
             if path is None:
                 raise error.CorruptedFileError('cannot find path for %s' % rev)
-            f.write(path + '\0')
+            f.write(path + b'\0')
         f.write(hsh)
 
     @staticmethod