--- a/scripts/desktop_l10n.py
+++ b/scripts/desktop_l10n.py
@@ -1281,15 +1281,15 @@ class DesktopSingleLocale(LocalesMixin, 
         # see bug
         funsize_info = {
             'locales': self.query_locales(),
             'branch': self.config['branch'],
             'appName': self.config['appName'],
             'platform': self.config['platform'],
         }
         self.info('funsize info: %s' % funsize_info)
-        self.set_buildbot_property('funsize_info', funsize_info,
+        self.set_buildbot_property('funsize_info', json.dumps(funsize_info),
                                    write_to_file=True)
 
 # main {{{
 if __name__ == '__main__':
     single_locale = DesktopSingleLocale()
     single_locale.run_and_exit()