Uncaught exception 'MongoException' with message 'Invalid object ID'

Uncaught exception 'MongoException' with message 'Invalid object ID'

I had a PHP code which was working nicely, but after I updated Mongo
(2.4.4) and mongo driver for php (1.4.2), my code started to generate
fatal error.
The line which generates an error is this one
$something = new MongoId($some);
It generates an error:
Fatal error: Uncaught exception 'MongoException' with message 'Invalid
object ID'
Surely, I can roll back my updates, but is there any idea how I can fix it
without rolling back?