How to add a field to a dbf file?

I have something about like this:

from dbfpy import dbf
import random

db = dbf.Dbf('DMWWGS84/DMAWGS84.dbf',new=False)
db.addField(("Data","D"))
for record in db:
    print record
    record["Data"]=random.random()
db.close()

But it complains:

Traceback (most recent call last):
  File "merge_csv.py", line 5, in <module>
    db.addField(("Data","D"))
  File "/Users/alex/anaconda2/lib/python2.7/site-packages/dbfpy/dbf.py", line 246, in addField
    raise TypeError("At least one record was added, "
TypeError: At least one record was added, structure can't be changed

What record is it talking about? Is this a good way to do this?