MySQL fails to add days to a date when month is over

I am trying to get a final date when a number of days (pplazo input) has elapsed, starting on a date (pfecha input), and avoiding to count certain dates (feriado) that are listed on a table. So far this is what i got to, using a Stored Procedure: Input parameters: pfecha -- DATE pplazo -- INT (11) PROC:BEGIN DECLARE i INT(1); START TRANSACTION; SET i=1; lazo:LOOP IF NOT EXISTS (SELECT * FROM feriados WHERE feriado=pfecha+i) THEN SET pfecha=pfecha+1; SET i=i+1; END IF; IF i=pplazo THEN LEAVE lazo; END IF; END LOOP lazo; COMMIT; SELECT pfecha as respuesta; END The thing is, when the days to count make the date go pass the end of the month, then the "respuesta" turns to 0000-00-00. This shouldn't be happening, if I input 15 days starting on the 2016-04-20 then the resulting date should be something like 2016-05-5. Can you spot my mistake? Could you point me in the right direction?