Odd csv column-name truncation with only one column

[Tim Chase]

tim at laptop:~/tmp$ python
Python 2.6.6 (r266:84292, Dec 26 2010, 22:31:48)
[GCC 4.4.5] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import csv
>>> from cStringIO import StringIO
>>> s = StringIO('Email\nfoo at example.com\nbar at example.org\n')
>>> s.seek(0)
>>> d = csv.Sniffer().sniff(s.read())
>>> s.seek(0)
>>> r = csv.DictReader(s, dialect=d)
>>> r.fieldnames
['Emai', '']

I get the same results using Python 3.1.3 (also readily available on
Debian Stable), as well as working directly on a file rather than a
StringIO.

Any reason I'm getting ['Emai', ''] (note the missing ell) instead
of ['Email'] as my resulting fieldnames?  Did I miss something in
the docs?

-tkc