NSDateFormatter dateFromString возвращает ноль

I have the following code:

NSDateFormatter *df = [[[NSDateFormatter alloc] init] autorelease];
    /*2010-11-02 20:31:39*/
[df setDateFormat:@"YYYY-MM-dd hh:mm:ss"];
NSDate* date = [df dateFromString:@"2010-11-02 20:31:39"];

date is nil.

Any idea why?