cant get quickcoverter to work on a margin

Aug 12 at 7:54 PM
hi all,
im tring to get a simple quickconverter bind to work in a margin propriety
im currently trying to do like this but its not working
Margin="{qc:Binding 'new Thickness(20)'}
the error says '' is not a valid value for Margin
thanks a lot,
Luca.