diff --git a/lib/internal/cluster/utils.js b/lib/internal/cluster/utils.js
index ba72ff90945d3c..94b82d039a098a 100644
--- a/lib/internal/cluster/utils.js
+++ b/lib/internal/cluster/utils.js
@@ -7,7 +7,7 @@ module.exports = {
   handles: {} // Used in tests.
 };
 
-const callbacks = {};
+const callbacks = new Map();
 var seq = 0;
 
 function sendHelper(proc, message, handle, cb) {
@@ -18,7 +18,7 @@ function sendHelper(proc, message, handle, cb) {
   message = util._extend({ cmd: 'NODE_CLUSTER' }, message);
 
   if (typeof cb === 'function')
-    callbacks[seq] = cb;
+    callbacks.set(seq, cb);
 
   message.seq = seq;
   seq += 1;
@@ -34,9 +34,13 @@ function internal(worker, cb) {
 
     var fn = cb;
 
-    if (message.ack !== undefined && callbacks[message.ack] !== undefined) {
-      fn = callbacks[message.ack];
-      delete callbacks[message.ack];
+    if (message.ack !== undefined) {
+      const callback = callbacks.get(message.ack);
+
+      if (callback !== undefined) {
+        fn = callback;
+        callbacks.delete(message.ack);
+      }
     }
 
     fn.apply(worker, arguments);