Create ZOHO expense issue

Create ZOHO expense issue

I am working on create expenses  on ZOHO, it always say   {"code":4,"message":"Invalid value passed for JSONString"}

My code 

$customFields=array();
$data_string = '{"account_id":"7817300000000xxxx","date":"2015-10-23","paid_through_account_id":"781730000000xxxxx","project_id":"","mileage_type":"","start_reading":"","end_reading":"","amount":"200","tax_id":"78173000000xxxxxx","is_inclusive_tax":"false","is_billable":false,"reference_number":"iqbal","description":"hello text testt","vendor_id":"781730000003xxxxx","customer_id":"781730000012xxxxx","currency_id":"781730000000xxxxx","exchange_rate":1,"custom_fields":[]}
';

       

        $ch = curl_init('https://books.zoho.com/api/v3/expenses?organization_id=35909461');
        curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
        curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($ch, CURLOPT_HTTPHEADER, array(
                'Content-Type: application/json',
                'Content: ' . $data_string)
        );

        $result = curl_exec($ch);

Please help its very urgent.

Thanks in advance