[Haskell-beginners] The last statement in a 'do' construct must be an expression: a <- readArray arr (1, 1)

I'm getting the above error message and I cannot figure out why? buildPair = do arr <- newArray ((1,1),(1,10)) 37 :: ST s (STArray s (Int,Int) Int) a <- readArray arr (1,1) writeArray arr (1,1) 64 b <- readArray arr (1,1) return (a,b) main = print $ runST buildPair -- -- Regards, KC