Paypal subscription error: The link you have used to enter the PayPal system is invalid. Please review the link and try again

I'm using Paypal website payment standard to subscription for my site. Here is the variables passing in Paypal which I dumped. I'm passing this values to 'https://www.sandbox.paypal.com/cgi-bin/webscr' using post method.

Field Name     Value
a3             19.99 
business       [email]
cancel_return  http://example.com
cmd            _xclick-subscriptions 
currency_code  USD 
custom         This is custom 
invoice        [invoice]
item_name      Ac开发者_JS百科count
item_number    21
no_note        1 
no_shipping    1 
notify_url     http://example.com/ipn
p3             1 
return         http://example.com/success 
rm             2 
sra            1 
src            1 
srt            1 
t3             D

Now the matter is this that the Paypal shows a error :

The link you have used to enter the PayPal system is invalid.

Please review the link and try again. Anyone knows how to fix this?

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Don't use srt = 1. Use srt to set the number of times the subscription should renew. 1 doesn't make any sense because a recurring payment that occurs one time is not a recurring payment. Just leave srt off if it is 1, and it will default to recur until someone cancels it.