使用NSData自定义编码，初始化但崩溃

我花了一些认真的时间来调试此问题，但我感到很沮丧。

我试图将自定义对象紧密打包到一起，NSData以节省将它们保存到磁盘时的空间。我已经为这些自定义对象实现了自定义编码和解码方法。当我打包和解压缩对象时，所有东西都会根据调试器正确初始化。代码运行后立即具有EXC_BAD_ACCESS code=1或code=2。

这是编码和解码：

-(instancetype) initWithData:(NSData *)data {
    self = [super init];
    if (self) {
        _memConstants = [[DWMemoryConstants alloc]init];

        int arraySize = _memConstants.chunkSize * _memConstants.chunkSize;
        NSData* unencodedBlocks[arraySize];
        [data getBytes:unencodedBlocks];

        _blocks = [[NSMutableDictionary alloc]init];
        for (int i = 0; i < 10; i++) {
            DWBlock *block = [[DWBlock alloc]initWithData:unencodedBlocks[i]];
            [_blocks setObject:block forKey:[NSValue valueWithCGPoint:CGPointFromString(block.blockName)]];

            if (i == 0) {
                _position = CGPointFromString(block.chunkName);
            }
        }
        _chunkName = NSStringFromCGPoint(_position);
        _bounds = CGRectMake(_position.x * _memConstants.chunkSize * kBlockSpriteWidth,
                             _position.y * _memConstants.chunkSize * kBlockSpriteWidth,
                             _memConstants.chunkSize * kBlockSpriteWidth,
                             _memConstants.chunkSize * kBlockSpriteWidth);
    }
    return self;
}

-(NSData *) customEncode {
    NSMutableData *data = [[NSMutableData alloc]init];

    NSData* blocks[_blocks.allValues.count];
    int count = 0;
    for (DWBlock *block in _blocks.allValues) {
        blocks[count] = [block customEncode];
        count++;
    }
    [data appendBytes:&blocks length:sizeof(blocks)];

    return data;
}

异常发生在此代码的末尾。它成功地首先打印了所有日志消息，并且我已经确认Chunk对象没有任何问题：

-(void) testEncodeDecode {
    DWChunk *testChunk = [[DWChunk alloc]initWithPosition:CGPointMake(100, 100)];
    NSData *testData = [testChunk customEncode];
    NSLog(@"datasize= %@", testData);
    DWChunk *unencodedChunk = [[DWChunk alloc]initWithData:testData];
    NSLog(@"blocks=%@", unencodedChunk.blocks.allValues);
    NSLog(@"this message will print");
}

就像我说的那样，Chunk和Block对象的所有变量都已正确初始化。我怀疑该问题与对象的不当释放有关，但我不知道从这里去哪里。

编辑：我已启用检查僵尸，现在我收到此错误消息：

2014-10-24 11:38:31.775 DigWorld[10622:60b] *** -[NSConcreteMutableData release]: message sent to deallocated instance 0x81790710