Partner could make a mistake

Playing with a regular partner, I am defending against 2H.

Partner leads the 6 of spades to my Ace. I return a low spade. Declarer takes it with the Queen under which partner plays the 5 of spades.  Declarer then plays a low heart to my Ace.

In a unbid suit, the 6 of spades should normally be from an honor.  But the 6-5 is what partner would play if he had a doubleton spade.  So ... what do you play when you are in with the Ace of hearts?

Well, partner has no spade honor. That's obvious, thanks to declarer playing the Queen of spades (King of spades would have been a better play).  He must have a doubleton!  I played a third spade.  It was too late now.  Declarer took the trick, pulled trumps and finessed clubs twice, discarding two diamonds, and losing only one spade, one heart, one diamond and one club.  2H+1 was a near-bottom board for us.

I could have devoted the problem a little more thought.  Even if partner has doubleton spades, declarer has a 5-card spade suit, and his minor suit losers will go away on them.  I need to switch.  But it is more likely that partner lead top of nothing from 6-5-2 or plumb made a mistake (that's what happened on this hand).  A minor suit switch is called for.