Saving contact custom fields giver invalid_field error

So I was trying to follow the API documentation to save a contact with custom fields.

Contact is success created with no custom fields. But when I provide custom fields to it,I get error.

$postData = array(
			"name"=> trim($name),
			"email" => trim($email),
			"phone" => trim($phone),
			"mobile" => trim($phone),
			"custom_fields" => array("lb_module_id" => $member_id,
															 "lb_user_id" => $user_id,
															 "lb_module" => $module)
		);
		$url = "https://".$this->domain.".freshdesk.com/api/v2/contacts";
		$this->curl->setOpt(CURLOPT_USERPWD, $this->apiKey.":x");
		$this->curl->setHeader("Content-Type", "application/json");
		$this->curl->post($url, json_encode($postData));
		if ($this->curl->error) {
			var_dump($this->curl->response);
			return 'Error: ' . $this->curl->errorCode . ': ' . $this->curl->errorMessage;
		}
		else {
			return $this->curl->response;
		}

 The error var_dump

object(stdClass)[2533]
  public 'description' => string 'Validation failed' (length=17)
  public 'errors' => 
    array (size=3)
      0 => 
        object(stdClass)[2484]
          public 'field' => string 'lbModuleId' (length=10)
          public 'message' => string 'Unexpected/invalid field in request' (length=35)
          public 'code' => string 'invalid_field' (length=13)
      1 => 
        object(stdClass)[2541]
          public 'field' => string 'lbUserId' (length=8)
          public 'message' => string 'Unexpected/invalid field in request' (length=35)
          public 'code' => string 'invalid_field' (length=13)
      2 => 
        object(stdClass)[2542]
          public 'field' => string 'lbModule' (length=8)
          public 'message' => string 'Unexpected/invalid field in request' (length=35)
          public 'code' => string 'invalid_field' (length=13)

 can someone guide me, what am I doing wrong here.