If you are using the most recent mailbox3 patch (I should have renamed it, sorry...I've no done so to make it clear) you should be getting an error message that tells you to use binary or Message.  So I don't understand how you are getting this message.  I still don't know what it is you are passing to the add method.

RC2 will be cut starting Sunday morning CET, and after that we will be making no further non-critical changes.  So if you've got bugs, best we find them before the end of the day tomorrow ;)