diff --git a/lib/index.js b/lib/index.js
index 46ad49c6..8caaa960 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -5,7 +5,11 @@
 'use strict';
 
 if (typeof Realm != 'undefined') {
+    // The global Realm constructor should be available on device (using JavaScriptCore).
     module.exports = Realm;  // eslint-disable-line no-undef
-} else {
+} else if (navigator.userAgent) {
+    // The userAgent will be defined when running in a browser (such as Chrome debugging mode).
     module.exports = require('./realm');
+} else {
+    throw new Error('Missing Realm constructor - please ensure RealmReact framework is included!');
 }