as3 externalinterface addcallback not working

Hi

sorry for my bad english.

i tryed to call function from java script to swf using 'addCallback', but its not working below my code

as3:

ExternalInterface.addCallback("myFun_2", aa);

function aa(va)

{

          txt.text = "working --- 2" + va;

}

JS:

calling_1('working...');

function calling_1(v)

                     {

                              alert(v);

                              getElementById('go').myFun_2(v);

                     }

here not the madatary to send the values, but testing perposs i took.

some one help me whats wrong i did.

thanks in advance

Chandu