Cannot retrieve the content of the Computed Field again.

I'm creating a "link" field and a computed field.
In computed I insert:
<?php $entity_field[0]['value'] =  array_pop(array_pop(field_get_items($entity_type, $entity, 'field_link'))); ?> 
The first time everything works. But all subsequent created fields that try to retrieve the content of the "link" field show a:0:{} instead of the desired URL, what could be the reason? 
 How can I correctly retrieve the content of the link field in the Computed Field? What might this be related to?