Sunroof won't close
 

'09 ML550. When I push the close-sunroof button, the roof moves all the way forward to about half an inch from its closed position, then reverses about four inches, as if it has encountered something that is stopping it from closing. Tap the button again and it closes all the way.

There is nothing visibly obstructing the roof, no obvious reason why it stops closing.

I can take it to the dealer to be fixed, but I just wondered if anybody else had had this problem and what the resolution was.