In v4.0, every getMore command calls serverGlobalParams.featureCompatibility.getVersion() .
However, this check trips an invariant if there is no FCV document in place yet. First-time launch of a mongod in a replica set does not populate the FCV document. That doesn't happen until the call to rs.initiate(), which leaves a period of time during which any getMore command will crash the server.
The check was removed for v4.2, so this bug should not occur in future releases.