使用NSImage时垃圾收集崩溃

这段代码是从我正在进行的一个项目中分离出来的。它在我的Mac OS 10.5.7上持续再现垃圾收集错误,有时还会崩溃。我已经研究它太久了,所以我的问题是:其他人看到为什么在垃圾收集时会出现错误了吗?

- (void) doCrash: (id) sender
{
    NSArray *lURLArray = [ NSArray arrayWithObjects:
                          @"http://userserve-ak.last.fm/serve/300x300/23621007.jpg",
                          @"http://userserve-ak.last.fm/serve/300x300/26675609.png",
                          @"http://userserve-ak.last.fm/serve/300x300/26675609.png",
                          nil ];
    NSString *lImageURL = nil;
    for (lImageURL in lURLArray)
    {
        NSImage *lImage = [[NSImage alloc] initWithContentsOfURL: [NSURL URLWithString: lImageURL]];

        NSSize targetSize = NSMakeSize(80,80);
        NSImage *newImage = [[NSImage alloc] initWithSize:targetSize];
        [newImage lockFocus];

        NSRect thumbnailRect = NSMakeRect(0,0,80,80);
        NSRect sourceRect = NSMakeRect(0,0,[lImage size].width,[lImage size].height);

        [lImage drawInRect: thumbnailRect
                  fromRect: sourceRect
                 operation: NSCompositeSourceOver
                  fraction: 1.0];                

        [newImage unlockFocus];
    }
}

当垃圾回收器释放其中一个映像时,会触发垃圾回收错误消息,如下所示:

reference count underflow for <address>, break on auto_refcount_underflow_error to debug.

谢谢,