[Typo3-dev] Userdefined field in $tca does not save...

[typo3 at stauffiger.ch]

Hello typo3-pro's

I've got a probem which I just can't solve on my own:

I've created a Extension using a type userFunc field in the $tca array.
I've programmed it like it's said to in the "TYPO3 Core
APIs"-Documentation.

It renders fine, but it simply won't save.

Does any of you have got a idea? (Because I've run out of..)

Thanks & have a nice day
Markus