Procedure passed Datetime parameter problem

[Thomas Gruener]

Hi,
I have a stored procedure in which I pass a datetime parameter. but for some reason it won't recognice it.
The parameter firstDate is passed, but when I try to set the new value for it I get either 'NULL' or '0000-00-00 00:00:00' as output for it. Why is that?

call sp_Month_Daily_Total_Count_Person(null);
I get: '0000-00-00 00:00:00', NULL as answer????

So how can I pass in a datetime value and change it like to the first of the month????

Here is my Code:
CREATE DEFINER=`root`@`%` PROCEDURE `sp_Month_Daily_Total_Count_Person`(IN firstDate DATETIME)
BEGIN
-- set date to first and last day of month
IF firstDate IS NULL THEN
SET firstDate := DATE_FORMAT(NOW(), '01-%m-%Y');
ELSE
SET firstDate := CONCAT(DATE_FORMAT(STR_TO_DATE(firstDate, '%d-%m-%Y'), '01-%m-%Y'));
END IF;

SET @last := CONCAT(DATE_FORMAT(LAST_DAY(STR_TO_DATE(firstDate, '%d-%m-%Y')), '%d-%m-%Y 23:59:59'));
SELECT firstDate AS First, @last as Last;
END


Thanks for any help.