Manual page CQ not correct mean on string


#1
> CREATE CONTINUOUS QUERY "cq_30m" ON "food_data" BEGIN
  SELECT mean("website") AS "mean_website",mean("phone") AS "mean_phone"
  INTO "a_year"."downsampled_orders"
  FROM "orders"
  GROUP BY time(30m)
END

If I do a mean on ‘string’ field, I am getting
ERR: unsupported mean iterator type: *query.stringInterruptIterator