objective-c - Why do these custom accessors cause EXC_BAD_ACCESS when trying to save managed object context?

Question

In my NSManagedObject subclass, I've written a set of custom accessor methods to expose a public CMTime property called videoDuration which is backed by an NSData attribute called videoDurationData...

- (CMTime)videoDuration
{
    [self willAccessValueForKey:@"videoDuration"];
    NSValue *videoDurationValue = [self primitiveVideoDuration];
    [self didAccessValueForKey:@"videoDuration"];
    if (nil == videoDurationValue) {
        NSData *videoDurationData = [self videoDurationData];
        if (nil != videoDurationData) {
            videoDurationValue = [NSValue valueWithCMTimeData:videoDurationData];
            [self setPrimitiveVideoDuration:videoDurationValue];
        }
    }

    return [videoDurationValue CMTimeValue];
}

- (void)setVideoDuration:(CMTime)videoDuration
{
    [self willChangeValueForKey:@"videoDuration"];
    NSValue *videoDurationValue = [NSValue valueWithCMTime:videoDuration];
    [self setPrimitiveVideoDuration:videoDurationValue];
    [self didChangeValueForKey:@"videoDuration"];
    [self setVideoDurationData:[NSData dataWithValue:videoDurationValue]];
}

- (NSValue *)primitiveVideoDuration
{
    NSData *videoDurationData = [self videoDurationData];
    if (nil != videoDurationData) {
        NSValue *videoDurationValue = [NSValue valueWithCMTimeData:videoDurationData];
        return videoDurationValue;
    }

    return nil;
}

- (void)setPrimitiveVideoDuration:(NSValue *)primitiveVideoDuration
{
    if (nil != primitiveVideoDuration) {
        NSData *videoDurationData = [NSData dataWithValue:primitiveVideoDuration];
        [self setVideoDurationData:videoDurationData];
    }
    else {
        [self setVideoDurationData:nil];
    }
}

After adding in undo/redo support, my app crashes on -[NSManagedObjectContext save:] with a EXC_BAD_ACCESS memory exception. However, if I comment out the primitive accessor methods (and have the -primitiveVideoDuration method simply return nil, then everything works as expected. I'm guessing that this has something to do with CMTime not being a key-value compliant scalar structure? (see update below)

UPDATE -- 5/8/12: This thread and the key-value coding programming guide seem to suggest that KVC now supports arbitrary struct data, which would lead me to believe that what I'm trying to do is indeed possible?

UPDATE 2 -- 5/8/12: Evidently, by simply not implementing the primitive accessors, everything works correctly. Why this is the case is still baffling to me...