diff --git a/lib/Connection.js b/lib/Connection.js
index 39db5f0..3f03685 100644
--- a/lib/Connection.js
+++ b/lib/Connection.js
@@ -379,7 +379,7 @@ Connection.prototype.openBox = function(name, readOnly, cb) {
   if (this.state !== 'authenticated')
     throw new Error('Not authenticated');
 
-  if (cb === undefined) {
+  if (typeof readOnly === 'function') {
     cb = readOnly;
     readOnly = false;
   }