Undefined index: type in replicator field

Hello! Adding a new set to a replicator field throws the following error:

Illuminate\Foundation\Bootstrap\HandleExceptions::handleError vendor/statamic/cms/src/Fieldtypes/Replicator.php:74

$fields = $this->fields($row['type'])->addValues($row)->preProcess()->values()->all();

Blueprint: events.yaml Section: replicator

      - 
        handle: perf_section
        field:
          display: Veranstaltungen
          type: section
          icon: section
          instructions: Performances
          listable: hidden
      -
        handle: performance_dates
        field:
          collapse: false
          sets:
            '':
              display: 'Veranstaltung / Performance'
              fields:
                -
                  handle: perf_date
                  field:
                    mode: single
                    time_enabled: true
                    time_required: true
                    earliest_date: '1900-01-01'
                    full_width: false
                    inline: false
                    columns: 1
                    rows: 1
                    display: Datum
                    type: date
                    icon: date
                    instructions: Date
                    listable: hidden
                    width: 50

Any ideas on what could cause the error?