Sticking to a plan

Playing matchpoints, West has opened 1S over which partner (North) overcalled 2D.  They land up in 4S and partner leads the Ace of diamonds and I see:

Dealer: W Vul: None	North ♠ ♥ ♦ ♣
West ♠ ♥ ♦ ♣		East ♠ AKxx ♥ xxxx ♦ x ♣ xxxx
	South ♠ x ♥ Axxxx ♦ Jxxxx ♣ xx
Bidding:	1S - 2D - 4S	HTML Bridge Hand Layout Creator

With a singleton on the board and declarer having a singleton, I pretty much have to show suit preference.  So I played the Jack of diamonds to tell partner about my heart honor.

Partner duly switched to the ten of hearts and I won with the Ace.  Now what?

Partner needs a club through, I thought, and led a club.  This was horribly wrong.  Can you see why?

The answer is in the card that partner led -- he led the 10 of hearts.  That denies KQJ of hearts.  Declarer must have those 3 hearts.  So, the ten must be a singleton!  I needed to give partner a ruff.