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What happens is that I have this ORA-01722 error despite verifying that my tables have only numeric values but it does not identify it problem what I want is to create a function that adds any column of a query with two tables

create table empleado1(
   cod    varchar2(5) not null,
   nombre varchar2(15) not null,
   sueldo FLOAT not null
);
create table empleado2(
   cod    varchar2(5) not null,
   nombre varchar2(15) not null,
   sueldo FLOAT not null
);

insert into EMPLEADO1 (COD,NOMBRE,SUELDO) values ('00001','Ricardo',100);
insert into EMPLEADO1 (COD,NOMBRE,SUELDO) values ('00002','Jorje',100);
insert into EMPLEADO2 (COD,NOMBRE,SUELDO) values ('00001','Freeman',100);
insert into EMPLEADO2 (COD,NOMBRE,SUELDO) values ('00002','Pepe',100);

CREATE FUNCTION SF_PRE1 (columna FLOAT)
RETURN FLOAT
AS
total FLOAT;
BEGIN
    SELECT SUM(columna) INTO total
    FROM empleado1
    INNER JOIN empleado2 ON empleado1.sueldo = empleado2.sueldo;
    RETURN total;
END;
SELECT SF_PRE1('sueldo')
FROM dual;
this is the code