Getting null values in an update statement

I want to perform an UPDATE to a field in a table. Such a field is of type
numeric, does not allow null values and has a default value of 0 (zero). The
situation I am facing is that the records that do not meet the update
criteria are set to null and I do not know why. I get an error message
saying that a violation has been made because the field cannot be set to
null. Then, I change the field definition so it accepts null values, run the
update statement and I confirm that such a field is set to null for the
records that do not meed the criteria. How can I avoid this sitution?
Does this behavior has to do with the way I am using the update statement?
Below is the UPDATE statement for your reference.
I appreciate your help in advance.

Regards,
Jorge Maldonado

UPDATE temp_lista_titulos SET tmt_porc =
(SELECT SUM(tmt_clave) AS suma FROM
(SELECT t1.tmt_clave, t1.tmt_album AS album
FROM temp_lista_titulos as t1, temp_lista_titulos as t2
WHERE t1.tmt_album = t2.tmt_album AND t1.tmt_clave != t2.tmt_clave) temp
WHERE tmt_album = album
GROUP BY tmt_album)