มีปัญหากับ PDO lastInsertId() ค่าออกเป็น 0 ไม่ออกเป็น id สุดท้าย

<?php
include 'connect.php';
$machine_id = htmlspecialchars($_POST['machine_id']);
$memtotal = htmlspecialchars($_POST['memtotal']);
$fullname = htmlspecialchars($_SESSION['fullname']);
try {
		$insertM = $db->prepare('INSERT INTO tbl_logs(machine_id,start,staff,member_total) VALUES (:machine_id,NOW(),:fullname,:memtotal)');
		$insertM->bindParam(':machine_id',$machine_id,PDO::PARAM_STR);
		$insertM->bindParam(':fullname',$fullname,PDO::PARAM_STR);
		$insertM->bindParam(':memtotal',$memtotal,PDO::PARAM_INT);
		$q = $insertM->execute();
		echo $lastId = $db->lastInsertId();
		if($q)
	{
		//echo "<script>alert('Save successfully!');</script>";
		echo "<script>window.top.window.showResult('1');</script>";
	}
		else
	{
	//echo "<script>alert('Error! Cannot save data');</script>";
			echo "<script>window.top.window.showResult('2');</script>";
	}
	}	
catch(PDOException $e)
    {
    	echo "Error: " . $e->getMessage();
    }
$db = null;
?>