Some commit messages contain unicode characters, making them strings.

scripts/slave/recipe_modules/auto_bisect/bisect_results.py

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import json
 import re
 
 # The perf dashboard looks for a string like "Estimated Confidence: 95%"
 # to decide whether or not to cc the author(s). If you change this, please
 # update the perf dashboard as well.
@@ skipping 122 matching lines @@
     body = ''
     if self.culprit_cl_hash:
       body += _RESULTS_REVISION_INFO % {
           'subject': self.culprit_subject,
           'author': self.culprit_author,
           'cl_date': self.culprit_date,
           'commit_info': self.commit_info,
           'cl': self.culprit_cl_hash
       }
     body += self._compose_revisions_table()
-    return body
+    return body.encode('ascii','replace')
 
   def _make_footer(self):
     return _RESULTS_THANKYOU
 
   def _gather_results(self):
     # TODO(robertocn): Add viewcl link here.
     # TODO(robertocn): Merge this into constructor.
     bisector = self._bisector
     config = bisector.bisect_config
 
@@ skipping 90 matching lines @@
   column_widths = [0] * len(data[0])
   for line in data:
     column_widths = [max(longest_len, len(prop)) for
                      longest_len, prop in zip(column_widths, line)]
   for line in data:
     for prop, width in zip(line, column_widths):
       result += prop.ljust(width + 1)
     result += '\n'
   return result