Issue saving a document in the shell when the document contains a reference with the element [$db]

The following code results in this error.

db.sellers.find().forEach(function(seller)

);

uncaught exception: field names cannot start with $ [$db]

I have signed the contributor agreement and issued a pull request for the patch to resolve this issue.
https://github.com/mongodb/mongo/pull/46

It affects versions mongo 1.8 and higher.