MongoDB upgrade 2.4 to 2.6 check returns error in config.changelog collection

In running the db.upgradeCheckAllDBs() command to check for issues in order to upgrade from MongoDB 2.4 to 2.6, an error is reported:

Checking collection config.changelog

Collection Error: lack of _id index on collection: config.changelog

I don't think the _id index check should occur on collections in the config database, right? This check is skipped for several other internal collections.

Is it safe to ignore this error and move forward with the upgrade if no other errors are reported?