Why is NSUserDefaults unwilling to save my NSDictionary?

Question

I'm using KVC to serialize an NSObject and attempt to save it to NSUserDefaults, which is giving me an Attempt to insert non-property value when I try to store my NSDictionary.

Following are the properties of the object in question, MyClass:

@interface MyClass : NSObject
@property (copy,nonatomic) NSNumber* value1;
@property (copy,nonatomic) NSNumber* value2;
@property (copy,nonatomic) NSString* value3;
@property (copy,nonatomic) NSString* value4;
@property (copy,nonatomic) NSString* value5;
@property (copy,nonatomic) NSString* value6;
@property (copy,nonatomic) NSString* value7;
@property (copy,nonatomic) NSString* value8;
@end

When it is time to save MyClass it occurs here:

-(void)saveMyClass
{
  NSArray* keys = [NSArray arrayWithObjects:
                @"value1",
                @"value2",
                @"value3",
                @"value4",
                @"value5",
                @"value6",
                @"value7",
                @"value8",
                nil];
  NSDictionary* dict = [self dictionaryWithValuesForKeys:keys];
  for( id key in [dict allKeys] )
  {
    NSLog(@"%@ %@",[key class],[[dict objectForKey:key] class]);
  }
  NSUserDefaults* defaults = [NSUserDefaults standardUserDefaults];
  [defaults setObject:dict forKey:[NSString stringWithString:kMyClassKey]];
  [defaults synchronize];
}

which produces this output:

2012-02-23 19:35:27.518 MyApp[10230:40b] __NSCFConstantString __NSCFNumber
2012-02-23 19:35:27.519 MyApp[10230:40b] __NSCFConstantString __NSCFNumber
2012-02-23 19:35:27.519 MyApp[10230:40b] __NSCFConstantString __NSCFString
2012-02-23 19:35:27.519 MyApp[10230:40b] __NSCFConstantString __NSCFString
2012-02-23 19:35:27.520 MyApp[10230:40b] __NSCFConstantString __NSCFString
2012-02-23 19:35:27.520 MyApp[10230:40b] __NSCFConstantString __NSCFString
2012-02-23 19:35:27.520 MyApp[10230:40b] __NSCFConstantString __NSCFString
2012-02-23 19:35:27.520 MyApp[10230:40b] __NSCFConstantString NSNull
2012-02-23 18:38:48.489 MyApp[9709:40b] *** -[NSUserDefaults setObject:forKey:]: Attempt to insert non-property value '{
    value1 = "http://www.google.com";
    value2 = "MyClassData";
    value3 = 8;
    value4 = "<null>";
    value5 = "http://www.google.com";
    value6 = 1;
    value7 = "http://www.google.com";
    value8 = 4SY8KcTSGeKuKs7s;
}' of class '__NSCFDictionary'.  Note that dictionaries and arrays in property lists must also contain only property values.`

As you can see, all of the objects in the dict are property list values and all of its keys are NSString*. What trivia am I lacking in order to execute this? Or should I give up and use writeToFile or similar?