KeyError in Aggregation.dict_rows()

[defgsus]

using parking-data export:

(search().terms("place_id", valid_place_ids)
    .agg_terms("place",field="place_id", size=1000)
    .agg_date_histogram("week", calendar_interval="week")
    .metric_avg("free", field="num_free")
    .pipeline_derivative("grad", buckets_path="free")
    .execute()
    .dict_rows()
)