diff --git a/git_config.py b/git_config.py
index a667b3f..aa07d1b 100644
--- a/git_config.py
+++ b/git_config.py
@@ -697,7 +697,7 @@
       self._Set('merge', self.merge)
 
     else:
-      fd = open(self._config.file, 'ab')
+      fd = open(self._config.file, 'a')
       try:
         fd.write('[branch "%s"]\n' % self.name)
         if self.remote: