field_name key issue

Hi,

When saving/updating a product with downloadable file attached, I get the following error

The following field has no field_name key
array ( 'fid' => '13893', 'uid' => '1', 'filename' => 'test.pdf', 'uri' => 'private://products/test.pdf', 'filemime' => 'application/pdf', 'filesize' => '154779289', 'status' => '1', 'timestamp' => '1498310402', 'origname' => 'test.pdf', 'type' => 'document', 'metadata' => array ( ), 'alt' => '', 'title' => '', 'display' => '1', 'description' => '', )

Any advise please to fix the issue?

Thanks

* PS: To find the above error I have used the following code in my module:

if(empty($field['field_name'])){
drupal_set_message('The following field has no field_name key');
drupal_set_message(''.var_export($field,1).'');
}