Tumble window for source ?

[sfu2]

I'm try to do a group by query based on tumble window and I ended up with this

create materialized view tumble_v as
  select avg(data) from tumble(records, created_at, interval '1' hour) as t
    group by t.window_start;

where records are a source from kafka. Then it gets

ERROR:  Catalog error: table not found: blocks

saying that tumble function can only be used on tables. But having all stream data loaded into risingwave seems un-efficient. Is there any work-around? Or are there any upcoming release that would make tumble support for source?