diff --git a/exercises/00-sqrt/go.go b/exercises/00-sqrt/go.go
index 4aa0bd2..6a9f8e9 100644
--- a/exercises/00-sqrt/go.go
+++ b/exercises/00-sqrt/go.go
@@ -7,7 +7,17 @@ import (
 
 const min_threshold = 1e-200
 
-func Sqrt(x float64, threshold float64) (z float64, iterations int) {
+type ErrNegativeSqrt float64
+
+func (e ErrNegativeSqrt) Error() string {
+	return fmt.Sprintf("cannot Sqrt negative number: %v", float64(e))
+}
+
+func Sqrt(x float64, threshold float64) (z float64, iterations int, err error) {
+	if x < 0 {
+		err = ErrNegativeSqrt(x)
+		return
+	}
 	z = x
 	old_z := z
 	for {
@@ -25,7 +35,10 @@ func main() {
 	threshold := 1.0
 	right_answer := math.Sqrt(2)
 	for threshold > min_threshold {
-		answer, iterations := Sqrt(2, threshold)
+		answer, iterations, err := Sqrt(2, threshold)
+		if err != nil {
+			panic(err)
+		}
 		fmt.Printf("%0.4g %0.4g %0.20f %0.20f %0.2d\n",
 			threshold, right_answer-answer, right_answer, answer,
 			iterations)