How to extend NSTextStorage?

According to documentation, I try to subclass NSTextStorage and use it in text view:

/*
 NSTextStorage is a semi-abstract subclass of NSMutableAttributedString. It
 implements change management (beginEditing/endEditing), verification of
 attributes, delegate handling, and layout management notification. The one
 aspect it does not implement is the actual attributed string storage --- this is
 left up to the subclassers, which need to override the two
 NSMutableAttributedString primitives:

 - (void)replaceCharactersInRange:(NSRange)range withString:(NSString *)str;
 - (void)setAttributes:(NSDictionary *)attrs range:(NSRange)range;

*/

So I try to use delegate, to handle all needed events with same functionality:

@implementation MyTextStorage

- (id) init {
    self = [super init];

    if (self != nil) {
        storage = [[NSMutableAttributedString alloc] init];
    }

    return self;
}

- (void) dealloc {
    [storage release];
    [super dealloc];
}

- (NSString *) string {
    return [storage string];
}

- (void) replaceCharactersInRange:(NSRange)range withString:(NSString *)str {
    [storage replaceCharactersInRange:range withString:str];
}

- (void)setAttributes:(NSDictionary *)attrs range:(NSRange)range {
    [storage setAttributes:attrs range:range];
}


@end

It is no matter what I'm use as delegate: NSTextStorage or NSMutableAttributedString, The result is the same:

An uncaught exception was raised * NSRunStorage, _NSBlockNumberForIndex(): index (18446744073709551615) beyond array bounds (0) * Terminating app due to uncaught exception 'NSRangeException', reason: '*** NSRunStorage, _NSBlockNumberForIndex(): index (18446744073709551615) beyond array bounds (0)'

Stack trace:

0   CoreFoundation                      0x00007fff840cd7b4 __exceptionPreprocess + 180
    1   libobjc.A.dylib