Error parsing pcap file

[bzImage]

$ captcp statistic 500M.pcap
# captcp 2010-2013 Hagen Paul Pfeifer and others (c)
# http://research.protocollabs.com/captcp/
Traceback (most recent call last):
  File "/usr/bin/captcp", line 6054, in <module>
    sys.exit(captcp.run())
  File "/usr/bin/captcp", line 6040, in run
    ret = classinstance.process_final()
  File "/usr/bin/captcp", line 4993, in process_final
    self.process_final_data()
  File "/usr/bin/captcp", line 4987, in process_final_data
    self.rexmt_final(connection.sc1)
  File "/usr/bin/captcp", line 4671, in rexmt_final
    sc.user_data["tl-ps-avg"]    = "%.2f" % numpy.mean(sc.user_data["_tl_pkt_sizes"])
AttributeError: 'NoneType' object has no attribute 'mean'