[Typo3-shop] Wrong parameters, GET/POST var 'tt_products' was missing

[Robert Markula]

This is the error message I get since I set the PIDitemDisplay in TS. 
What is the reason for this? In the manual there is no hint on how this 
setting might be related to this error. Looking at the URL, it did not 
change except for the PID of course.
Is it a bug or a feature? :)
And is there a way to get rid of this problem without tampering with the 
source?

Regards,
Ro