Getting "ERROR: upper bound of FOR loop cannot be null" when trying to use a stored proc in a check constraint def.

This stored procedure ...

create or replace function validate_proj_csv (proj_csv varchar)
returns int
language plpgsql
as
$$

-- This function used in a check constraint in the public.projects table to ensure that
-- all projects in column sibling_project_csv are valid projects.

DECLARE
  proj_arr varchar[];
  see_prj int;
 
BEGIN

  proj_arr := regexp_split_to_array(proj_csv,',');
 
  for x in 1 .. array_upper(proj_arr,1)
  loop
    select 1 into see_prj from public.projects where project = proj_arr[x];
    if (see_prj is null) then
      raise notice 'Project "%" in project csv "%" is not a valid project.', proj_arr[x],proj_csv;
      return 0;
    end if;

  end loop;
   
  return 1;

END;
$$ ;

... works fine...

dvdb=# select validate_proj_csv('sabin,strix2,ipu1.0');
 validate_proj_csv
-------------------
                 1
(1 row)

dvdb=# select validate_proj_csv('sabin,strix2,ipu1.00');
NOTICE:  Project "ipu1.00" in project csv "sabin,strix2,ipu1.00" is not a valid project.
 validate_proj_csv
-------------------
                 0
(1 row)

But when I try to use it in a check constraint....

dvdb=# alter table projects add constraint validate_sibling_project_csv check (validate_proj_csv(sibling_project_csv) = 0);
ERROR:  upper bound of FOR loop cannot be null
CONTEXT:  PL/pgSQL function validate_proj_csv(character varying) line 14 at FOR with integer loop variable

What's going on ?

How to get this to work ?