PDO not inserting - what's wrong..?

PDO not inserting - what's wrong..?

I'm stumped as to why it's not inserting into the database...
PHP Code:
$insert_user = $db->prepare("INSERT INTO `users(`deposit_address`,
`date_assigned`) VALUES (:deposit_address, :date_assigned)");
$insert_user->execute(array(':deposit_address' =>
$_SESSION['deposit_address'], ':date_assigned'=> time() ));
I did a var_dump and it returned:
object(PDOStatement)#3 (1) { ["queryString"]=> string(96) "INSERT INTO
`users(`deposit_address`, `date_assigned`) VALUES (:deposit_address,
:date_assigned)" }