I wrote some code to execute a query on my db. My code is like follow

My_Controller

class MY_Controller extends CI_Controller
{
    var $data;
    var $header_data;
    var $topmenu_data; 
    var $leftmenu_data;
    var $rightmenu_data;
    var $middle_data;
    var $footer_data;
 
    function __construct($props = array())
    {
        parent::__construct($props);
        $this->data   = array();
        $this->header_data = array();
        $this->topmenu_data = array();
        $this->leftmenu_data= array();
        $this->rightmenu_data= array();
        $this->middle_data = array();
        $this->footer_data = array();
        $this->errorbox_data= array();
    }
}

my groups controller

class Groups extends My_Controller
{

    function __construct()
    { 
        parent::__construct();
        $this->load->model('model_group');
        $this->load->model('model_post_comment');
        $this->load->model('model_event');
    }

    function create_group()
    {
        $userId = 100;
        $groupName = "TEST";
        
        $data = array('user_id' => $userId,
            'group_name' => $groupName,
            'db_add_date' => date('Y-m-d H:i:s'),
            'db_update_date' => date('Y-m-d H:i:s')
        );

        $r = $this->model_group->group_saveusers($data);
    
        $array=array("result"=>$r);
         
        echo json_encode($array);
    }
} 

here is my model

class Model_group extends CI_Model
{
    function __construct()
    {
        // Call the Model constructor
        parent::__construct();
    }

    function group_saveusers($group_data)
    {
        $this->db->insert_batch('my_table',$group_data);
        if ($this->db->affected_rows() > 0) {
            $group_id = $this->db->insert_id();
        }
        return $group_id;
    }
} 

Invoking the url below

http://mydomain/groups/create_group/

I get the error below

A PHP Error was encountered
Severity: Warning
Message: array_keys() [function.array-keys]: The first argument should be an array
Filename: database/DB_active_rec.php
Line Number: 1109

Am I missing something?