Smarty array with null value

Question

I'm trying to get the value of one position of array in a smarty .tpl, here's my code

{$destination = $destination_id}
{$bid = $destination}
{$shippingRate = $shipping.rates.$bid|@json_encode}                            
$dest: {$destination}
<br>
$bid: {$bid}
<br>
$shippingRate: {$shippingRate}

HTML result:

$dest: 244
$bid: 244
$shippingRate: null

And this is an example with the var $bid hardcoded:

{$destination = $destination_id}
{$bid = 244}
{$shippingRate = $shipping.rates.$bid|@json_encode}                            
$dest: {$destination}
<br>
$bid: {$bid}
<br>
$shippingRate: {$shippingRate}

HTML result:

$dest: 244
$bid: 244
$shippingRate: {"destination_id":"244","status":"A"}

I guess that I'm doing something wrong in the assignation of $bid, I did try with the following options with the same result:
{$bid = $destination|intval}
{$bid = $destination|strip|intval}
{$bid = $destination|strip}