diff --git a/offlineimap/folder/IMAP.py b/offlineimap/folder/IMAP.py
index 1fa35f7..3c702f4 100644
--- a/offlineimap/folder/IMAP.py
+++ b/offlineimap/folder/IMAP.py
@@ -506,7 +506,7 @@ class IMAPFolder(BaseFolder):
                 return
             result = imapobj.uid('store', '%d' % uid, 'FLAGS',
                                  imaputil.flagsmaildir2imap(flags))
-            assert result[0] == 'OK', 'Error with store: ' + '. '.join(r[1])
+            assert result[0] == 'OK', 'Error with store: ' + '. '.join(result[1])
         finally:
             self.imapserver.releaseconnection(imapobj)
         result = result[1][0]