Unable to remove element's tooltip


First of all - thanks for this great product, it just fits all our needs!

There is a small issue though... I want to be able to remove element's tooltip on specific case, but did not seem to have any success.
I tried to do it by passing null value to SetToolTip method, as I was used to in System.Windows.Controls tooltip, like this: ToolTipService.SetToolTip(element, null). it's not working, and the element still has a tooltip.

Do you have any other idea? it's quite urgent...

Thanks a lot in advance.