PostgreSQL plpgsql函数问题

我的PostgreSQL 16.1数据库中有一个运行在Debian 12.2上的函数:

CREATE OR REPLACE FUNCTION ref.lookup_xxx(
    in_code character varying,
    in_description character varying)
    RETURNS integer
    LANGUAGE 'plpgsql'
    COST 100
    VOLATILE PARALLEL UNSAFE
AS $BODY$
  declare id_val integer;
  begin
    if in_code is null then -- nothing to do
      return null;
    end if;
    -- check if code is already present in the table:
    id_val = (select min(id) from ref.xxx where code = in_code);
    if id_val is null then -- insert new code, desc into reference table:
      insert into ref.xxx (code, description) values (in_code, in_description) returning id_val;
    end if;
    return id_val; -- return id of new or existing row
  exception
    when others then
      raise exception 'lookup_xxx error code, desc = %, %', in_code, in_description;
  end; 
$BODY$;

它返回一个错误:

ERROR:  lookup_xxx error code, desc = 966501, <NULL>
CONTEXT:  PL/pgSQL function ref.lookup_xxx(character varying,character varying) line 15 at RAISE 

SQL state: P0001

如果我运行下面的特殊查询，它会成功:

insert into ref.xxx (code, description) values ('966501', null);

我无法运行此即席查询-它可能无法运行:

do $$
declare x integer;
begin
  insert into ref.xxx (code, description) values ('966501', null) returning x;
  raise notice 'x is %', x;
end; 
$$

我正在寻找任何建议来纠正这个功能-我已经判断了Postgres文档，但没有找到任何有用的东西.单步执行调试器中的函数会显示它在INSERT语句中失败.我在其他plpgsql函数中也有类似的查询，它们工作正常.