diff --git a/test/heap-test-coils.js b/test/heap-test-coils.js
index 5e11d92..0b1f5cb 100644
--- a/test/heap-test-coils.js
+++ b/test/heap-test-coils.js
@@ -11,7 +11,7 @@ const client = modbus.client.tcp.complete({
 client.connect()
 
 client.on('connect', function () {
-  for (let i = 1; i < 1e5; i++) {
+  for (var i = 1; i < 1e5; i++) {
     client.readCoils(0, 13)
   }