Cannot setup Dynamic Data from a database table for Drop Down field

I am lost… I try to set up Dynamic Data from a database table
I followed the description:
How do I build a select drop-down, radio button or checkbox group?
Created: Wednesday, 31 October 2012 04:39

The Debugger shows me Data:
Array
(
[Wv62bComprofiler] => Array
(
[0] => Array
(
[user_id] => 10002
[firstname] =>
[middlename] =>
[lastname] => admin_0T
)

[1] => Array
(
[user_id] => 1
[firstname] => Hansueli
[middlename] => juhr
[lastname] => Rickli
)

……


The db_multi_record_loader:
SELECT `user_id`, `firstname`, `middlename`, `lastname` FROM `wv62b_comprofiler` AS `Wv62bComprofiler`

The Drop Down: Dynamic Data:
Enable: Yes
Data Patth: Wv62bComprofiler
Value Key: user_id
Text Key: lastname

What do I miss?????

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