"imagecopy() expects parameter 1 to be resource, boolean given" Codeigniter

Question

I'm calling do_upload_img() function only if there is any input value, like this:

function xxx()
{
   if($_FILES['portfolioimg']['size'])
   {
       $portfolioimg = $this->do_upload_img();
   }

   // remaining code.......
}  

do_upload_img() function

function do_upload_img()
{
    if(isset($_FILES['portfolioimg']['size']) != 0){
       $name_array = array();
       $files = $_FILES;
       $cpt = count($_FILES['portfolioimg']['name']);

       $config['wm_type'] = 'overlay';
       $config['wm_overlay_path'] = './assets/img/watermark.png';
       $config['quality'] = 50;
       $config['wm_vrt_alignment'] = 'middle';
       $config['wm_hor_alignment'] = 'center';

       for($i=0; $i<=$cpt-1; $i++)
       {           
           $_FILES['userfile']['name']= $files['portfolioimg']['name'][$i];
           $_FILES['userfile']['type']= $files['portfolioimg']['type'][$i];
           $_FILES['userfile']['tmp_name']= $files['portfolioimg']['tmp_name'][$i];
           $_FILES['userfile']['error']= $files['portfolioimg']['error'][$i];
           $_FILES['userfile']['size']= $files['portfolioimg']['size'][$i];    

           $imgnam = rand(10,1000000);
           $ext = pathinfo($files['portfolioimg']['name'][$i], PATHINFO_EXTENSION);
           if($this->upload->initialize($this->set_upload_options($imgnam))){
               $data = $this->upload->do_upload();
               $config['source_image'] = './assets/portfolio_img/' . 'Soooo_'.$imgnam.'.'.$ext;
               $this->image_lib->initialize($config);
               $this->image_lib->watermark(); 
               $name_array[] = $this->upload->data('file_name');
           } else {
               $this->session->set_flashdata('falsemsg','Only jpg, jpeg, png files are allowed to be uploaded.');
           }

       } 
       return $name_array;
    }else{
      return false;
    }
}

View

<input class="fileUpload" accept="image/jpeg, image/jpg, image/png, image/gif" name="portfolioimg[]" type="file" value="" data-msg-accept="Please upload only jpg, jpeg, png and gif file"> 

If I run this xxx() function and if the portfolioimg input field have no value then it showing a warning message like this:

A PHP Error was encountered
Severity: Warning
Message: imagecopy() expects parameter 1 to be resource, boolean
given
Filename: libraries/Image_lib.php
Line Number: 1217

And also if I use error_reporting(0), still the warning is displaying.