NSDate doesn't return value

In my iPhone app I am converting NSString into NSDate. One string is working fine and returns proper NSDate, but similar kind of another string returns (null). Why this happens?

NSDateFormatter *df =[[NSDateFormatter alloc]init];
df.dateFormat=@"dd MM yyyy";
NSDate *nextMonthDay=[df dateFromString:@"29 01 2015"];


NSLog(@"nextMonthDay  %@",nextMonthDay);
// Returns ---  nextMonthDay  2015-01-28 18:30:00 +0000

NSDate *nextMonthDay2=[df dateFromString:@"29 02 2015"];

NSLog(@"nextMonthDay  %@",nextMonthDay2);
// Returns ---  nextMonthDay  (null)

Please help me to fix this. Thanks in advance.