CGContext无效上下文0x0

Question

当我使用UIGraphicsBeginImageContextWithOptions来初始化上下文，然后将图像绘制到该内容时，我会收到 代码如下：

        NSString* tempPath = ...;
        int count = 5;
        CGFloat width = 320;
        CGFloat height = 480;
        CGFloat scale = 2.0;
        CGFloat contextScale = 1.0;
        UIGraphicsBeginImageContextWithOptions(CGSizeMake(width * scale, height * scale * count),
                                               NO, contextScale);
        int i = 0;
        //link picture
        while (count > 0) {
            NSString* imgName = [NSString stringWithFormat:@"img_%d.jpg", i];
            NSString* imgFile = [tempPath stringByAppendingPathComponent:imgName];
            UIImage* tempImg = [[UIImage alloc] initWithContentsOfFile:imgFile];

            [tempImg drawInRect:CGRectMake(0, height * scale * i,
                                           width * scale, height * scale)];

            --count;
            ++i;
        }
        UIImage* linkedImg = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();

错误发生在：[tempImg drawInRect：.....]

我无法理解为什么。 帮助我。