diff --git a/lib/cluster.js b/lib/cluster.js
index 175645f713f701..68a772a74a9b59 100644
--- a/lib/cluster.js
+++ b/lib/cluster.js
@@ -749,7 +749,7 @@ function internal(worker, cb) {
   return function(message, handle) {
     if (message.cmd !== 'NODE_CLUSTER') return;
     var fn = cb;
-    if (message.ack !== undefined) {
+    if (message.ack !== undefined && callbacks[message.ack] !== undefined) {
       fn = callbacks[message.ack];
       delete callbacks[message.ack];
     }
diff --git a/test/parallel/test-cluster-invalid-message.js b/test/parallel/test-cluster-invalid-message.js
new file mode 100644
index 00000000000000..006c80a7aae8fc
--- /dev/null
+++ b/test/parallel/test-cluster-invalid-message.js
@@ -0,0 +1,22 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const cluster = require('cluster');
+
+if (cluster.isMaster) {
+  const worker = cluster.fork();
+
+  worker.on('exit', common.mustCall((code, signal) => {
+    assert.strictEqual(code, 0);
+    assert.strictEqual(signal, null);
+  }));
+
+  worker.on('online', () => {
+    worker.send({
+      cmd: 'NODE_CLUSTER',
+      ack: -1
+    }, () => {
+      worker.disconnect();
+    });
+  });
+}