an exception thrown on ScrollViewer - Value doesn't fall within the expected range

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Silverlight

scrolling

Hi,
I would like to have a scrollbar around grid which is embedded in paneldragdroptarget control. I would say that this should work

<paneldragdroptarget...settings>
 <scrollviewer ....>
  <grid....>
  </grid>
 </scrollviewer>
</paneldragdroptarget>

But I'm getting an exception thrown on ScrollViewer - Value doesn't fall within the expected range.
Any idea what could be the reason? I have tried tried different settings, different order,...but nothing.
Thank you