How to change text field text using if/else if string is NULL or 0?

Below is my code:

if(PREREQUISITEstring == (NSString *)[NSNull null] || [PREREQUISITEstring isEqual: @"0"]) {
  PREREQUISITEfield.text = @"2";
} else { 
  PREREQUISITEfield.text = PREREQUISITEstring;
}

I am 100% sure that PREREQUISITEstring is "0" and it shows as "0" when I'm debugging but when it comes to runtime, that string doesn't change to 2, it stays as 0.