Notice (8): Undefined index: id?

What does that mean, and where is the problem?
Notice (8): Undefined index: id [CORE/cake/libs/model/model.php, line
1281...on line 77

That's line 77:
if ($this->User->save ( $this->data, true , array ( 'firstname',
'lastname', '....' ))) {
... }

What's wrong?

thx
Aurelius
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