macOS I can't get fast-enumeration to work?

#import <Foundation/Foundation.h>
 
@protocol Talker
 
- (void) talk;
 
@end

#import <Foundation/Foundation.h>
#import "Talker.h"
 
@interface Chatter : NSObject < Talker > {
  NSString *phrase;
}
 
- (void) setPhrase: (NSString *) phrase;
 
@end

#import <Foundation/Foundation.h>
#import "Talker.h"
 
@interface Chatterbox : NSObject < Talker > {
  NSMutableArray *talkers;
}
 
- (void) addTalker: (id < Talker >) talker;
 
@end

#import <Foundation/Foundation.h>
#import "Chatter.h"
 
@implementation Chatter
 
- (void) setPhrase: (NSString *) newPhrase {
  phrase = newPhrase;
}
 
- (void) talk {
  printf("%s\n", [phrase cStringUsingEncoding: NSUTF8StringEncoding]);
}
 
@end

#import <Foundation/Foundation.h>
#import "Chatterbox.h"
#import "Talker.h"
 
@implementation Chatterbox
 
- (void) addTalker: (id < Talker >) talker {
  [talkers addObject: talker];
}
 
- (void) talk {
  //Something doesn't work here... it's not iterating
  for (id < Talker > *talker in talkers) {
    [talker talk];
  }
}
 
@end

#import <Foundation/Foundation.h>
#import "Chatter.h";
#import "Chatterbox.h"
 
int main(Void) {
  
  NSAutoreleasePool *autoreleasepool = [[NSAutoreleasePool alloc] init];
  
  Chatterbox *chatterbox = [[Chatterbox alloc] init];
  
  Chatter *c1 = [[Chatter alloc] init];
  [c1 setPhrase: @"Hello Melbourne!"];
  [chatterbox addTalker: c1];
  
  Chatter *c2 = [[Chatter alloc] init];
  [c2 setPhrase: @"Hello Australia!"];
  [chatterbox addTalker: c2];
  
  Chatter *c3 = [[Chatter alloc] init];
  [c3 setPhrase: @"Hello World!"];
  [chatterbox addTalker: c3];
  
  Chatter *c4 = [[Chatter alloc] init];
  [c4 setPhrase: @"Hello Universe!"];
  [chatterbox addTalker: c4];
  
  [chatterbox talk];
  
  [c1 release];
  [c2 release];
  [c3 release];
  [c4 release];
  
  [autoreleasepool release];
  
}