FileConveyor找不到sftp transporter

Question

# python /root/src/fileconveyor/fileconveyor/arbitrator.py 
/root/src/fileconveyor/fileconveyor/filter.py:10: DeprecationWarning: the sets module is deprecated
  from sets import Set, ImmutableSet
2014-01-23 00:23:34,615 - Arbitrator                - WARNING  - File Conveyor is initializing.
2014-01-23 00:23:34,621 - Arbitrator                - WARNING  - Loaded config file.
2014-01-23 00:23:34,624 - Arbitrator                - ERROR    - The transporter module 'sftp' could not be found. Tried (sftp, transporters.transporter_sftp)
TransporterAvailabilityTestError Consult the log file for details

我刚安装了fileconveyor并创建了一个简单的config.xml，但系统启动时会因为无法找到sftp transporter而窒息。我已经验证了sftp传输器在那里并具有我认为的良好权限和所有权（我是root，文件归root和644所有）。

我做错了什么？