diff --git a/src/nelderMead.js b/src/nelderMead.js
index fe38e91..1cc99b6 100644
--- a/src/nelderMead.js
+++ b/src/nelderMead.js
@@ -129,7 +129,7 @@ export function nelderMead(f, x0, parameters) {
                 if (sigma >= 1) break;
 
                 // do a reduction
-                for (i = 1; i < simplex.length; ++i) {
+                for (let i = 1; i < simplex.length; ++i) {
                     weightedSum(simplex[i], 1 - sigma, simplex[0], sigma, simplex[i]);
                     simplex[i].fx = f(simplex[i]);
                 }