Why this haskell code does not print the error message?

Code:

---

sqrtInt' :: Int -> Int
sqrtInt' 0 = 0
sqrtInt' n
        | n < 0
                = error "sqrt': negative argument"
        | (m + 1)^2 == n
                = m + 1
        | otherwise
                = m
        where m = sqrtInt' (n-1)

---

thanks...