error: db type could not be determined

[neutrinman at myrealbox.com]

why does the following error occur?

def quit_time():
	data_file = shelve.open("data.dat", "c")
	data_file["quit_time"] = datetime.datetime.today()
	print data_file["quit_time"]
	raw_input("enter")


Traceback (most recent call last):
  File "D:\a\TEXT\Py\going_over\going_over.py", line 192, in -toplevel-
    main()
  File "D:\a\TEXT\Py\going_over\going_over.py", line 190, in main
    quit_time()
  File "D:\a\TEXT\Py\going_over\going_over.py", line 156, in quit_time
    data_file = shelve.open("data.dat", "c")
  File "D:\A\UTILITIES\PYTHON\PYTHON23J\lib\shelve.py", line 231, in
open
    return DbfilenameShelf(filename, flag, protocol, writeback, binary)
  File "D:\A\UTILITIES\PYTHON\PYTHON23J\lib\shelve.py", line 212, in
__init__
    Shelf.__init__(self, anydbm.open(filename, flag), protocol,
writeback, binary)
  File "D:\A\UTILITIES\PYTHON\PYTHON23J\lib\anydbm.py", line 80, in
open
    raise error, "db type could not be determined"
error: db type could not be determined