trillspan

[E Appeldoorn]

I wanted to move a TrillSpann downwards to within the bar on the middle 
barline.
I finaly came up with
\once \override TrillSpanner.Y-offset = #-2 \startTrillSpan a4*5 
\stopTrillSpan
But got an error 'unbound TrillSpannEvent' and nothing has changed.
So how do I move that TrillSpanner southwards?
(version 22.2.1)
Thanks.
vriendelijke groet, Erik Appeldoorn