You’re sitting West, and you pick up a nice looking hand:

Partner makes a limit raise to 3S, guaranteeing 4 spades and 10-12 HCP.
You cross your fingers and trot out Roman Keycard Blackwood (0314), but partner rains on your parade by bidding 5C.
You then sign off in 5S, praying for a minor miracle to save you from going down one.
But partner now shocks you by bidding 6S:

What’s going on? And what do you do now?