issue returning a json Id variable

I am guessing this is going to be a huge oversight on my part but i have an issue with json that i havent been able to resolve..

i am using the following code to run an update/insert query

$.post($("#updateEnquiryform").attr("action"),$("#updateEnquiryform").serialize(),function(){
	
}).success(function(response){
	alert(response);
	alert(response.lastId);
},"json");

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my php statement returns a response with the following :

$result->execute();
$insertedID = $pdo->lastInsertId();
$returnUrl = array ("lastId"=>$insertedID);
echo json_encode($returnUrl);

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Essentially, all that i want to do is to return the last inserted ID whenever i have a new record. at the moment, when i alert the response, i get {"lastId":"0"}.. but when i try response.lastId it returns undefined ?

I'm not sure where i'm going wrong or if i'm even doing this with the right approach ?

Thanks