iOS Convert NSString to NSDate alway nil

I'm trying to convert string to nsdate. The string date is "12/27/2014 07:42:00 AM +0000"

Here is my code:

NSDate *date =[[NSDate alloc] init];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
dateFormatter.dateFormat = @"MM/dd/yyyy HH:mm:ss a";
date = [ dateFormatter dateFromString:dateString];

The date value is always nil. Any of you knows what canI be doing wrong?

I'll really appreaciate your help.