API retrieving Customer unknown error 1060

[Nico12]

hi, i try to retrieve a customer over the api. it works if i use the login name for the filter. if i use the "owner-id" i get a error message. i dont really understand the error message and in the api documentation its not documented. the customer exists, i fetched the owner-id over the api.

my request:
<?xml version="1.0" encoding="utf-8"?>
<packet version="1.6.6.0"><customer><get><filter><owner-id>6</owner-id></filter><dataset><gen_info/></dataset></get></customer></packet>

the response (errcode 1060 specified owner can not have sub objects):
<?xml version="1.0" encoding="UTF-8"?>
<packet version="1.6.6.0"><customer><get><result><status>error</status><errcode>1060</errcode><errtext>Specified owner can not have sub objects</errtext><filter-id>6</filter-id></result></get></customer></packet>

OS: Ubuntu 12.04 LTS
Plesk Version: 12

any ideas ?