MongoMapper - Mongo::OperationFailure - Database command 'update' failed: BSON field 'update.multi' is an unknown field

I am getting Mongo::OperationFailure - Database command 'update' failed: BSON field 'update.multi' is an unknown field. error on MongoMapper set method.

eg: Metadata.set({:cid => 123}, :brand => "b1")

REF: http://ift.tt/pYduRz

I am using following environment:
- Ruby 2.2.0
- Rails 3.2.22
- Gem MongoMapper 0.14.0
- MongoDB 3.6.0