When mongod 3.4 attempts to join a replica set with catchUpTimeoutMillis:-1, it fails to join the replica set and logs the error message "BadValue: Invalid value in field "catchUpTimeoutMillis": -1: catch-up timeout must be greater than or equal to 0". -1 is the default value for catchUpTimeoutMillis on 3.6, so this is likely to occur when adding a 3.4 mongod to 3.6 replica set. The error message should explain that in order to add a 3.4 mongod to a 3.6 replica set, the catchUpTimeoutMillis value for the replica set must be changed.