[TYPO3-mvc] Unable to get pid of Domain Model Object

[Alexander Dick]

Hi List, 

I am unable to get the pid of a record. In my Model there is the property and the corresponding getter: 

/**	
 * @var integer
 */
protected $pid;

/** 
 * @return integer
 */
public function getPid() {
    return $this->pid;
}

but getPid returns always null - what's the problem there? 
getUid works as expected. 

Thanks in advance
Alex