iPhone - linker error when using constants

Why does I have a linker error on this :

- (void) reverseGeocoder:(MKReverseGeocoder *)geocoder didFailWithError:(NSError *)error {

    if ([[error domain] isEqualToString:MKErrorDomain] && [error code] == MKErrorPlacemarkNotFound) {
          Do things
    }
}


Undefined symbols for architecture i386:
  "_MKErrorDomain", referenced from:
      -[MapController reverseGeocoder:didFailWithError:] in MapController.o
ld: symbol(s) not found for architecture i386
collect2: ld 开发者_如何学Creturned 1 exit status

MapKit framework is included. MapKit/MapKit.h is imported. Everything works fine with MapKit calls when I do not have those lines...

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The same question was answered here before. Hope it's going to help you.