Javascrip error in Paypal

Hello, I'm doing some tests using the sandbox. I've configured the seller account to redirect clients back to the seller's website automatically. I know this automatic redirect only works if clients pay with a paypal account and not with their credit card. I've sucessfully tried several tests paying with paypal test accounts but when I try to pay with credit card the payment is ok (validated + verfiied), but in the last page (Thank you for your payment blah blah... and link back to the seller's website) I get the following Javascript error (I've just noticed now that I've tested on IE).

 
This is the error as shown in the Firefox error console:

 
Error: echo is not defined
Archivo de origen: https://www.sandbox.paypal.com/es/cgi-bin/webscr?cmd=_flow&SESSION=bhEeL7Uuty-4rMUma50SaKoeKIn55dUlc2B-TN3ocQKG4Kkgik4lHb7xiXi&dispatch=50a222a57771920b6a3d7b606239e4d529b525e0b7e69bf0224adecfb0124e9bd7371532d7fbae4715b6ee2d3a381319d43ea0c93be06bb1
Línea: 37

 
and it points to this (exactly to the echo):


YAHOO.util.Event.addListener(window,"load",function(){
try {
PAYPAL.common.appendField("done", "flow_name", PAYPAL.common.loginflow);
PAYPAL.bp.init("done","bp_mid");PAYPAL.common.appendField("done", "bp_ks1");
PAYPAL.ks.init("done","password","bp_ks2");
PAYPAL.ks.init("done","password-confirm","bp_ks3");
} catch(err){ echo(err); }
});