Help with a query

Hi list!

I have a query that worked some years with my "old" Debian 10 and
Postgres 11.
Now with Debian 12 and Postgres 15 throws an error...

So, my problem: I need to create a table with the _physical_ boundaries
of the cities.
I already asked you some years ago and now I have my query.

So I create a Table `city_boundaries`. Now I must "compact" it, in order
to have only the outer polygon.
The table has a column `way` defined as geometry(MultiPolygon,3857).

As suggested 2021, I aggregate the building using 4 temporary columns,
wayn1, wayn2, wayn3 and wayn4. They are defined too as
geometry(MultiPolygon,3857).

Then I run my queries:

UPDATE city_boundaries SET wayn1 =
ST_Multi(ST_SimplifyPreserveTopology(way, 100));
UPDATE city_boundaries SET wayn2 = ST_Multi(ST_Buffer(wayn1, 50,
'join=miter'));
UPDATE city_boundaries SET wayn3 = ST_Multi(ST_Buffer(wayn2, -50,
'join=miter'));
UPDATE city_boundaries SET wayn4 = ST_Makevalid(wayn3);

The last query throws the error:

FEHLER:  Geometry type (GeometryCollection) does not match column type
(MultiPolygon)

I suppose, in the last versions of PostGIS the Function ST_Makevalid
will return something other than in the previous one.
Can someone suggest me how to "convert" it as needed?

Thanks a lot
Luca Bertoncello
([email protected])