TrailDBConstructor crashes if the first field is called username

[rhymes]

I don't know why but it seems to me that TrailDBConstructor crashes if the first fieldname is username:

Python 2.7.13 (default, May 2 2017, 15:58:16)
[GCC 4.2.1 Compatible Apple LLVM 8.1.0 (clang-802.0.42)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

from traildb import TrailDBConstructor, TrailDB
cons = TrailDBConstructor('foobar', ['username'])
[1] 88728 abort python

I noticed it because I was going through the tutorial and the first example didn't work http://traildb.io/docs/tutorial/#part-i-create-a-simple-traildb

I noticed that it doesn't happen if the field called "username" is not the first item in the array:

from traildb import TrailDBConstructor
TrailDBConstructor('foobar', ['alice', 'username'])
<traildb.traildb.TrailDBConstructor object at 0x10f609450>