Entity Metadata Wrapper: Setting User Reference
Looks like Drupal is trying to set the delta column to the string "uid", which suggests you need another level in your array:

$user_perm[] = array (
  'uid'    =&gt; $account-&gt;uid,
  'access' =&gt; TRUE,
);

$ewrapper-&gt;field_user_permission-&gt;set($user_perm);