GET followers/ids and remove deleted users


I retrieve +750.000 users from my customer account using the followers/ids and custom developed scripts.
Then i retrieve with users/lookup the detail info for those accounts.

Once the first leg is done, i start again only with the new users, so keep in my db the users and can do some stat work.

The big problem is about api followers/ids is returning non existent users or error users. When i call the users/lookup call with any, some or all of the users at list having non existent users, it throws a
[message] => Sorry, that page does not exist
[code] => 34
If i discard manually those id’s, i can keep calling my software and update users.

Why twitter api followers/ids keep returning non existent users and how can i passthought it?