diff --git a/bench/cec2020/basicFunctions.go b/bench/cec2020/basicFunctions.go
index 8fa5479..3880fe1 100644
--- a/bench/cec2020/basicFunctions.go
+++ b/bench/cec2020/basicFunctions.go
@@ -70,7 +70,8 @@ func Griewank(x []float64) float64 {
 
 	for i := range x {
 		sum += math.Pow(x[i], 2) / 4000
-		prod -= math.Cos(x[i] / math.Sqrt(float64(i)))
+		// use i+1 in sqrt so that we don't get Nan.
+		prod *= math.Cos(x[i] / math.Sqrt(float64(i+1)))
 	}
 
 	return sum - prod + 1