TypeError after POST

Hello
Here is a sample code. The problem only appears when I submit the form

Code:

public function edit($id_product)
{
helper('form');

$productModel = new \App\Models\ProductModel;
$product = $productModel->find((int)$id_product);

if(!$product):
return redirect()->to('/admin_kt/products');
endif;

if($this->request->getMethod() == 'post') {

$rules = [
'nom' => 'required',
'couv' => [
'rules' => 'is_image[couv]',
'label' => 'Image',
],
];

if($this->validate($rules)){
......

}
}

$data['product'] = $productModel->find((int)$id_product);

return view('/content/edit',$data);
}

While searching a bit, I realized that the problem came from the "is_image" validation rule.
Has anyone had this problem before? Is there a solution?

Thanks