diff --git a/lib/internal/cluster/child.js b/lib/internal/cluster/child.js
index 39e891214ff787..05cb6d25e81a9d 100644
--- a/lib/internal/cluster/child.js
+++ b/lib/internal/cluster/child.js
@@ -8,7 +8,7 @@ const Worker = require('internal/cluster/worker');
 const { internal, sendHelper } = require('internal/cluster/utils');
 const cluster = new EventEmitter();
 const handles = {};
-const indexes = {};
+const indexes = new Map();
 const noop = () => {};
 
 module.exports = cluster;
@@ -62,14 +62,18 @@ cluster._getServer = function(obj, options, cb) {
                       options.addressType,
                       options.fd ].join(':');
 
-  if (indexes[indexesKey] === undefined)
-    indexes[indexesKey] = 0;
+  let index = indexes.get(indexesKey);
+
+  if (index === undefined)
+    index = 0;
   else
-    indexes[indexesKey]++;
+    index++;
+
+  indexes.set(indexesKey, index);
 
   const message = util._extend({
     act: 'queryServer',
-    index: indexes[indexesKey],
+    index,
     data: null
   }, options);
 
@@ -108,7 +112,7 @@ function shared(message, handle, indexesKey, cb) {
   handle.close = function() {
     send({ act: 'close', key });
     delete handles[key];
-    delete indexes[indexesKey];
+    indexes.delete(indexesKey);
     return close.apply(this, arguments);
   }.bind(handle);
   assert(handles[key] === undefined);
@@ -141,7 +145,7 @@ function rr(message, indexesKey, cb) {
 
     send({ act: 'close', key });
     delete handles[key];
-    delete indexes[indexesKey];
+    indexes.delete(indexesKey);
     key = undefined;
   }