When adding a classify session to table:
PUT /restconf/config/vpp-classifier:vpp-classifier/classify-table/acl_table_test/classify-session/00:00:00:00:00:00:01:02:03:04:05:06:00:00:00:00

{"classify-session": [

{"hit_next": "permit", "match": "00:00:00:00:00:00:01:02:03:04:05:06:00:00:00:00", "opaque-index": "1", "advance": 1}

]}
The request fails with IllegalArgumentException, seems to be caused by the presence of opaque-index in the configuration.