Prelude.head: empty list in if statement

I'm writing a function and it throws Prelude.head empty list if I use the variable newBoard where I have sortBoard in the if-else it throws this error. If I use the original board character it doesn't throw this error.

foo :: [[Char]] -> [(Int, Int)] -> [(Int, (Int, Int))] -> Int -> [(Int,Int)]-> [Int]-> [[Char]]
foo [] _ _ _ _ _ = []
foo board [] _ _ _ [] = board
foo board spot array number startArray move = nextStep
    where 
        newBoard        = set board (head spot) ('1')
        tailSpot        = tail spot
        nextStep = 
            if tailSpot == []
                then if (tail move) == []
                        then newBoard
                    else sortBoard newBoard array 1 [] (tail move)
            else (foo newBoard tailSpot array 1 [] move)

For explination on this code, I have a game board and need to enter '1' in certain spots which are given by the variable spot. If I have gone through all the spots(tailSpot) and there are no more sections I need to change(move) it should just return the new board. If I still have sections I need to change, I exit this function and go back to finding the spots in that section so I can come back and switch them. If I still have a section and spots left, I keep going through this function until all spots are changed.