diff --git a/src/allmydata/frontends/ftpd.py b/src/allmydata/frontends/ftpd.py
index bba29160b..51330a78b 100644
--- a/src/allmydata/frontends/ftpd.py
+++ b/src/allmydata/frontends/ftpd.py
@@ -289,7 +289,7 @@ class FTPServer(service.MultiService):
         # make sure we're using a patched Twisted that uses IWriteFile.close:
         # see docs/frontends/FTP-and-SFTP.txt and
         # http://twistedmatrix.com/trac/ticket/3462 for details.
-        assert "close" in ftp.IWriteFile.names(), "your twisted is lacking"
+        assert "close" in ftp.IWriteFile.names(), "your twisted is lacking a vital patch, see docs/frontends/FTP-and-SFTP.txt"
 
         r = Dispatcher(client)
         p = portal.Portal(r)