can't start carbon

Hi I have a graphite. but I got error when I try to start carbon: /opt/graphite/bin/carbon-cache.py start

Traceback (most recent call last):
  File "bin/carbon-cache.py", line 32, in <module>
    run_twistd_plugin(__file__)
  File "/opt/graphite/lib/carbon/util.py", line 95, in run_twistd_plugin
    config.parseOptions(twistd_options)
  File "/usr/local/lib/python2.7/dist-packages/twisted/application/app.py", line 624, in parseOptions
    usage.Options.parseOptions(self, options)
  File "/usr/local/lib/python2.7/dist-packages/twisted/python/usage.py", line 267, in parseOptions
    self.subOptions.parseOptions(rest)
  File "/usr/local/lib/python2.7/dist-packages/twisted/python/usage.py", line 277, in parseOptions
    self.postOptions()
  File "/opt/graphite/lib/carbon/conf.py", line 214, in postOptions
    pwd.getpwnam(settings.USER)[2:4])
KeyError: 'getpwnam(): name not found: carbon'