Problem with calling function from dll

[deepblu at poczta.fm]

I have problem with using function from dll.

import ctypes

b = ctypes.windll.LoadLibrary("kernel32")

a = ""

b.GetComputerNameA(a,20)


But I got exception:

Traceback (most recent call last):
  File "<pyshell#323>", line 1, in <module>
    b.GetComputerNameA(a,20)
WindowsError: exception: access violation reading 0x00000014

Even when I write:
a = ctypes.c_char_p('.' * 20)
I got the same result.

Here I found this function description:
http://sd2cx1.webring.org/l/rd?ring=pbring;id=15;url=http%3A%2F%2Fwww.astercity.net%2F~azakrze3%2Fhtml%2Fwin32_api_functios.html


Please help.