diff --git a/benchmarks/experimental/async.js b/benchmarks/experimental/async.js
index ab2e19e..dd64cbb 100644
--- a/benchmarks/experimental/async.js
+++ b/benchmarks/experimental/async.js
@@ -9,7 +9,7 @@ app.experimental = true;
 
 // number of middleware
 
-const n = parseInt(process.env.MW || '1', 10);
+let n = parseInt(process.env.MW || '1', 10);
 console.log(`  ${n} async middleware`);
 
 while (n--) {
diff --git a/benchmarks/middleware.js b/benchmarks/middleware.js
index b60beb6..600f6f3 100644
--- a/benchmarks/middleware.js
+++ b/benchmarks/middleware.js
@@ -7,7 +7,7 @@ const app = koa();
 
 // number of middleware
 
-const n = parseInt(process.env.MW || '1', 10);
+let n = parseInt(process.env.MW || '1', 10);
 console.log(`  ${n} middleware`);
 
 while (n--) {