WampServer

I have the following lines of code
$stmt =$mysqli->prepare("INSERT INTO tbl VALUES (?,?)";
$stmt->bind_param('ss',$a,$b);

It works perfectly on wamp 1.5 but not on wamp 1.6. It says that the no of parameters does not match but i think everything is alright and it should be so because it does work on the earlier version of wamp. Can anyonehelp me please?