ADC stops reading on 510th iteration on Python

I wrote a basic test code to try the functionality of the python mraa class on my edison arduino board. The problem that I run into is adc function stops reading when the iteration arrives 510th level. Why do this problem occur?

Test Code:

import mraa

import time

i=0

print (mraa.getVersion())

while i<520:

        i=i+1

        try:

                time.sleep(0.5)

                print(i),

                print (mraa.Aio(0).read())

        except:

                print ("Are you sure you have an ADC?")

*****************************************

Output of the test code.

root@myedison:~# python test1.py

v0.5.4-5-g1ee0548

1 1019

2 1023

3 1023

.....

508 1023

509 1023

510 Are you sure you have an ADC?

511 Are you sure you have an ADC?