diff --git a/lib/internal/cluster/master.js b/lib/internal/cluster/master.js
index 457b3579b12c1c..2eead0b8a3a70a 100644
--- a/lib/internal/cluster/master.js
+++ b/lib/internal/cluster/master.js
@@ -148,7 +148,7 @@ function removeWorker(worker) {
   delete cluster.workers[worker.id];
 
   if (keys(cluster.workers).length === 0) {
-    assert(keys(handles).length === 0, 'Resource leak detected.');
+    assert(handles.size === 0, 'Resource leak detected.');
     intercom.emit('disconnect');
   }
 }
@@ -156,12 +156,10 @@ function removeWorker(worker) {
 function removeHandlesForWorker(worker) {
   assert(worker);
 
-  for (var key in handles) {
-    const handle = handles[key];
-
+  handles.forEach((handle, key) => {
     if (handle.remove(worker))
-      delete handles[key];
-  }
+      handles.delete(key);
+  });
 }
 
 cluster.fork = function(env) {
@@ -277,7 +275,7 @@ function queryServer(worker, message) {
 
   const key = `${message.address}:${message.port}:${message.addressType}:` +
               `${message.fd}:${message.index}`;
-  var handle = handles[key];
+  var handle = handles.get(key);
 
   if (handle === undefined) {
     let address = message.address;
@@ -302,12 +300,13 @@ function queryServer(worker, message) {
       constructor = SharedHandle;
     }
 
-    handles[key] = handle = new constructor(key,
-                                            address,
-                                            message.port,
-                                            message.addressType,
-                                            message.fd,
-                                            message.flags);
+    handle = new constructor(key,
+                             address,
+                             message.port,
+                             message.addressType,
+                             message.fd,
+                             message.flags);
+    handles.set(key, handle);
   }
 
   if (!handle.data)
@@ -319,11 +318,11 @@ function queryServer(worker, message) {
       errno: errno,
       key: key,
       ack: message.seq,
-      data: handles[key].data
+      data: handles.get(key).data
     }, reply);
 
     if (errno)
-      delete handles[key];  // Gives other workers a chance to retry.
+      handles.delete(key);  // Gives other workers a chance to retry.
 
     send(worker, reply, handle);
   });
@@ -346,10 +345,10 @@ function listening(worker, message) {
 // removed by a prior call to removeHandlesForWorker() so guard against that.
 function close(worker, message) {
   const key = message.key;
-  const handle = handles[key];
+  const handle = handles.get(key);
 
   if (handle && handle.remove(worker))
-    delete handles[key];
+    handles.delete(key);
 }
 
 function send(worker, message, handle, cb) {
diff --git a/lib/internal/cluster/utils.js b/lib/internal/cluster/utils.js
index 94b82d039a098a..c3e14cbb53a721 100644
--- a/lib/internal/cluster/utils.js
+++ b/lib/internal/cluster/utils.js
@@ -4,7 +4,7 @@ const util = require('util');
 module.exports = {
   sendHelper,
   internal,
-  handles: {} // Used in tests.
+  handles: new Map() // Used in tests.
 };
 
 const callbacks = new Map();