diff --git a/test/node-unit/cli/run-helpers.spec.js b/test/node-unit/cli/run-helpers.spec.js
index e12b97b875..e56cdb2868 100644
--- a/test/node-unit/cli/run-helpers.spec.js
+++ b/test/node-unit/cli/run-helpers.spec.js
@@ -1,6 +1,7 @@
 'use strict';
 
 const {validateLegacyPlugin, list} = require('../../../lib/cli/run-helpers');
+const Mocha = require('../../../lib/mocha');
 
 describe('helpers', function() {
   describe('validateLegacyPlugin()', function() {
@@ -56,6 +57,19 @@ describe('helpers', function() {
       });
     });
 
+    describe('when used with a third-party interface', function() {
+      it('should add the interface to "Mocha.interfaces"', function() {
+        // let's suppose that `glob` is an interface
+        validateLegacyPlugin(
+          {interface: 'glob'},
+          'interface',
+          Mocha.interfaces
+        );
+        expect(Mocha.interfaces, 'to satisfy', {glob: require('glob')});
+        delete Mocha.interfaces.glob;
+      });
+    });
+
     describe('when a plugin throws an exception upon load', function() {
       it('should fail and report the original error', function() {
         expect(