Bug in Mailman version 2.1.5

We're sorry, we hit a bug!

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Traceback:

Traceback (most recent call last):
  File "/home/mailman/scripts/driver", line 80, in run_main
    pkg = __import__('Mailman.Cgi', globals(), locals(), [scriptname])
  File "/home/mailman/Mailman/Cgi/listinfo.py", line 27, in 
    from Mailman import MailList
  File "/home/mailman/Mailman/MailList.py", line 49, in 
    from Mailman.Archiver import Archiver
  File "/home/mailman/Mailman/Archiver/__init__.py", line 17, in 
    from Archiver import *
  File "/home/mailman/Mailman/Archiver/Archiver.py", line 31, in 
    from Mailman import Mailbox
  File "/home/mailman/Mailman/Mailbox.py", line 21, in 
    import mailbox
  File "/usr/lib64/python2.6/mailbox.py", line 18, in 
    import email.message
ImportError: No module named message



Python information:

VariableValue
sys.version 2.6.6 (r266:84292, Aug 18 2016, 15:13:37) [GCC 4.4.7 20120313 (Red Hat 4.4.7-17)]
sys.executable /usr/bin/python
sys.prefix /usr
sys.exec_prefix /usr
sys.path /usr
sys.platform linux2


Environment variables:

VariableValue
SERVER_SOFTWARE Apache/2.2.15 (CentOS)
SCRIPT_NAME /mailman/listinfo
HTTP_IF_MODIFIED_SINCE Fri, 22 Sep 2017 15:10:31 GMT
SERVER_SIGNATURE
Apache/2.2.15 (CentOS) Server at mailman.connactivity.com Port 80
REQUEST_METHOD GET
PATH_INFO /lost-chords
SERVER_PROTOCOL HTTP/1.0
QUERY_STRING
HTTP_USER_AGENT CCBot/2.0 (http://commoncrawl.org/faq/)
SERVER_NAME mailman.connactivity.com
REMOTE_ADDR 54.198.246.116
PATH_TRANSLATED /home/mailman/public_html/lost-chords
SERVER_PORT 80
SERVER_ADDR 149.56.254.165
DOCUMENT_ROOT /home/mailman/public_html
PYTHONPATH /home/mailman
SCRIPT_FILENAME /home/mailman/cgi-bin/listinfo
SERVER_ADMIN support@connactivity.com
HTTP_HOST mailman.connactivity.com
REQUEST_URI /mailman/listinfo/lost-chords
HTTP_ACCEPT text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
GATEWAY_INTERFACE CGI/1.1
REMOTE_PORT 54930
HTTP_ACCEPT_ENCODING x-gzip, gzip, deflate