MySQL中的简单STR_TO_DATE问题

I am getting in a date like "Dec 2012" and want to turn it into a date like "2012-12-01". I am using STR_TO_DATE('Dec 2012','%b %Y') and it ends up looking like 2012-12-00. Any ideas how I can get it to set it to the first day of the month?I am getting in a date like "Dec 2012" and want