Why this haskell code does not print the error message?

[brack]

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...