Add Invoice CRM API

Add Invoice CRM API

Hey guys, 

There seems to be very little documentation about inserting invoice through API in ZOho CRM. I am trying to push this data:

$fields['data'][0]= array(
  "Subject"=>"Development Work",
  "Invoice Date"=>date('Y-m-d'),
  "Account Name"=>'NetON Test Account',
  "Contact Name"=>'Suman Dahal',
  "Status"=>'Created',
  "Product_Details"=> array(
    array(
      'product'=>
        array(
          "name"=>'Development Support Hours'
        ),
      "quantity"=>"0.5",
      "unit_price"=>"120",
      "list_price"=>"120"
  )
)
);
$post_fields = json_encode($fields);

And I am getting a weird error:
{"data":[{"code":"INVALID_DATA","details":{"expected_data_type":"long","api_name":"product","index":0,"parent_api_name":"Product_Details"},"message":"invalid data","status":"error"}]}

Can you let me know the documentation around it, or how I can fix this.