TYPO3 Solr: How to add additional fields to indexer?

Question

[TYPO3 8.7, ext_solr 9.0]

I have a custom extension for events. Solr should index the field startdate but doesn't. Everything else works fine.

plugin.tx_solr.index.queue {
    events = 1
    events {
        table = tx_customevents_domain_model_event
        fields {
            title = title
            abstract = teaser
            startdate_intS = startdate
            content = SOLR_CONTENT
            content {
                field = bodytext
            }
            url = TEXT
            url {
                typolink {
                    parameter =  {$plugin.tx_customevents_list.settings.singelePid}
                    additionalParams = &tx_customevents_list[event]={field:uid}&tx_customevents_list[action]=show&tx_customevents_list[controller]=Event
                    additionalParams.insertData = 1
                    returnLast = url
                    useCacheHash = 1
                }
            }
        }
    }
}
plugin.tx_solr.search.query.queryFields := addToList(startdate_intS^1.0)

The Fluid debugger does not show it in the resultSet object, even after having emptied the index queue.

What am I missing here?