NSAttributedString as a Transformable Attribute in Core Data Entity

In the Core Data Programming Guide under "Transformable Attributes" here. It is stated "You can now use the attribute as you would any other standard attribute, as illustrated in the following code fragment:"

The follow line of code is listed newEmployee.favoriteColor = [NSColor redColor];

I am trying to do something similar in my code but it doesn't work. Here is my code:

In Entry.h:

@interface Entry :  NSManagedObject  
{
}

@property (nonatomic, retain) NSAttributedString * contentString;
@property (nonatomic, retain) NSDate * date;
@property (nonatomic, retain) NSManagedObject * journal;

@end

and in Entry.m:

@implementation Entry 

@dynamic contentString;
@dynamic date;
@dynamic journal;

- (void)awakeFromInsert {
   [super awakeFromInsert];
   self.date = [NSDate date];
}

@end

Elsewhere I have this code:

Entry *newEntry =
    [NSEntityDescription insertNewObjectForEntityFor开发者_运维知识库Name:@"Entry"
        inManagedObjectContext:[self managedObjectContext]];
newEntry.contentString = [[[NSAttributedString alloc] initWithString:@"MyString"] autorelease];
newEntry.journal = self.journal;

The line

newEntry.contentString = [[[NSAttributedString alloc] initWithString:@"MyString"] autorelease];

is equivalent to

newEmployee.favoriteColor = [NSColor redColor];

As far as I can tell however this doesn't work. I get a runtime error:

2010-10-13 09:02:08.577 JournalMaker[2437:a0f] Cannot create NSData from object MyString{ } of class NSConcreteAttributedString

I have no idea what this error message is supposed to mean. Can someone explain?