salt: file.managed doesn't work with ldap users

scenario: user samu comes from an ldap:

$id samu
uid=10000(samu) gid=10000(utenti)

let’s have this testing.sls file:

/home/samu/testing:
  file.managed:
    - source: salt://testing
    - mode: 640
    - user: samu
    - group: utenti

and let’s run it, it will give you the following error:

$salt '*' state.highstate testing.sls
----------
    State: - file
    Name:      /home/samu/testing
    Function:  managed
        Result:    False
        Comment:   User samu is not available Group utenti is not available
        Changes: