A recent commit to master made it an error to pass a null value for the finalize field in a mapReduce command. The error is:
Previous versions of mongod treated this the same as a missing finalize field.
Unfortunately, all existing releases of the Java driver trigger this error when there is no finalize function specified by the application. While we could easily update later releases to non include the field instead of sending a null value for it, any existing application that uses a current driver and updates their server will run into this.
Please consider continuing to allow a null value for finalize.
The driver treats the query, sort, and scope fields similarly: it passes a null value if the application doesn't provide values. ]