diff --git a/lib/utils.js b/lib/utils.js
index ea7c8dbf..4ebd34f4 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -3,12 +3,14 @@
 const { Transform, Writable } = require('stream')
 
 function validateTopic (topic, message) {
+  if (!topic || topic.length === 0) { // [MQTT-3.8.3-3]
+    return new Error('impossible to ' + message + ' to an empty topic')
+  }
+
   const end = topic.length - 1
   const endMinus = end - 1
   const slashInPreEnd = endMinus > 0 && topic.charCodeAt(endMinus) !== 47
-  if (topic.length === 0) { // [MQTT-3.8.3-3]
-    return new Error('impossible to ' + message + ' to an empty topic')
-  }
+
   for (let i = 0; i < topic.length; i++) {
     switch (topic.charCodeAt(i)) {
       case 35: { // #
diff --git a/test/topics.js b/test/topics.js
index 69fa2c5a..7f28cde3 100644
--- a/test/topics.js
+++ b/test/topics.js
@@ -3,6 +3,14 @@
 const { test } = require('tap')
 const { setup, connect, subscribe } = require('./helper')
 const aedes = require('../')
+const { validateTopic } = require('../lib/utils')
+
+test('validation of `null` topic', function (t) {
+  // issue #780
+  t.plan(1)
+  const err = validateTopic(null, 'SUBSCRIBE')
+  t.equal(err.message, 'impossible to SUBSCRIBE to an empty topic')
+})
 
 // [MQTT-4.7.1-3]
 test('Single-level wildcard should match empty level', function (t) {