diff --git a/mode/mbox.py b/mode/mbox.py
index c229234..99025d9 100644
--- a/mode/mbox.py
+++ b/mode/mbox.py
@@ -52,7 +52,7 @@ class MboxMsgBuffer(Buffer):
     def _get_lines(self):
         d = {}
         for name in ('delivered-to', 'to', 'from', 'subject', 'date'):
-            s = self.msg.get(name)
+            s = self.msg.get(name, '')
             d[name] = cont_re.sub(' ', s.replace('\t', '    '))
         lines = [
             'Delivered-To: ' + d['delivered-to'],