Hello fellow Haskellers!

I'm approaching learning the basics of Haskell by going through https://wiki.haskell.org/99_questions. At the same time I write tests for my code in Hspec. Consider question no. 2: "Find the last but one element of a list".

My solution:

-- Problem 02
myButLast :: [a] -> a
myButLast [] = error "Empty list"
myButLast [x] = error "List has only one element"
myButLast [x1,x2] = x1
myButLast (x:xs) = myButLast xs

and a a test:

    describe "02 myButLast" $ do
        it "returns the last but one element of a list" $ do
            myButLast [] `shouldThrow` anyErrorCall
            myButLast [1] `shouldThrow` anyErrorCall -- <- this line causes the problem
            myButLast [1..4] `shouldBe` 3
            myButLast ['x','y','z'] `shouldBe` 'y'
            myButLast "abc" `shouldBe` 'b'

Building tests with stack test command causes the below compilation error:

    • No instance for (Num (IO a0)) arising from the literal ‘1’
    • In the expression: 1
      In the first argument of ‘myButLast’, namely ‘[1]’
      In the first argument of ‘shouldThrow’, namely ‘myButLast [1]’
   |
27 |             myButLast [1] `shouldThrow` anyErrorCall
   |

From what I understand, type of myButLast [1] is different than expected by shouldThrow. What I don't understand is why exactly it behaves so and how to fix this problem. Only that one assertion doesn't compile. The others are fine. Particularly, why does myButLast [] `shouldThrow` anyErrorCall work but with one element it doesn't?

Can you please give me a hand?