Error Upgrading from 5.6 to 6.6


I'm exploring upgrading from 5.6.3 (with basic x-pack) to 6.6.2 (basic x-pack built-in) and am seeing an odd error on the master node as the new 6.6.2 master tries to join:

java.lang.IllegalArgumentException: Unknown NamedWriteable [org.elasticsearch.cluster.metadata.MetaData$Custom][ml]

Given that we deploy elasticsearch with machine learning disabled why is this happening? I have no x-pack setting in the elasticsearch.yml of the new node joining the cluster.