Loop headaches

[dbrewerton]

Hello folks. I'm kinda stumped here so I could use a second pair of eyes. I'm trying to increment the x_device_id variable to the next ID in the database. MySQL is complaining about my incrementing the ID for some stupid reason. Here is what I have:

DELIMITER $$
DROP PROCEDURE IF EXISTS `monthly_btus` $$
CREATE PROCEDURE `monthly_btus`()

BEGIN

DECLARE x_device_id bigint;
DECLARE startdate TIMESTAMP;
DECLARE monthenddate TIMESTAMP;
declare my_slice bigint;
declare monthly_btus bigint;
declare lastdate timestamp;

select device_id from ewise_device limit 1 into x_device_id;

main_loop: LOOP

if (x_device_id IS NULL) then
  LEAVE main_loop;
end if;

set startdate = (SELECT last_day( NOW( ) - INTERVAL 2 MONTH )); # Gets last timestamp of 2 months ago
set monthenddate = (SELECT DATE_FORMAT(NOW() ,'%Y-%m-01')); # Gets last timestamp for last month

SELECT slice_id FROM `ewise_slice` WHERE device_id = x_device_id AND slice_timestamp BETWEEN startdate AND monthenddate ORDER BY slice_id DESC LIMIT 1 INTO my_slice;

SELECT slice_timestamp FROM `ewise_slice` WHERE device_id = x_device_id AND slice_timestamp BETWEEN startdate AND monthenddate ORDER BY slice_id DESC LIMIT 1 INTO lastdate;

select cumul_btus from ewise_cumulative_btus where slice_id = my_slice INTO monthly_btus;

INSERT into ewise_arch_btus (device_id,enddate,totalbtus) values(x_device_id,lastdate,monthly_btus)
set x_device_id = x_device_id + 1;

END LOOP main_loop;
END $$

DELIMITER ;

Call me crazy but I really don't see why it doesn't like it. I took out my select echo statements so the code isn't so long.