出现这个问题，我去google了下有两种解决办法：

错误出在：；fast-rcnn/lib/datasets/VOCdevkit-matlab-wrapper/voc_eval.m中的

for i = 1:length(VOCopts.classes)
  cls = VOCopts.classes{i};
  res(i) = voc_eval_cls(cls, VOCopts, comp_id, output_dir, rm_res);
end

1.删除原有的cache

2.In VOCCode, inside the VOCdevkit folder, there will be a file called VOCinit.m .
In that, on around line 31, change the VOC.testset to 'test' instead of 'val'

3.第三个是我自己想的办法，也是最有效的。

In VOCCode, inside the VOCdevkit folder, there will be a file called VOCinit.m ，就是在VOCcode中有个VOCinit.m,找到第81行，也就是这行：

VOCopts.classes={...
        'aeroplane'
        'bicycle'
        'bird'
        'boat'
        'bottle'
        'bus'
        'car'
        'cat'
        'chair'
        'cow'
        'diningtable'
        'dog'
        'horse'
        'motorbike'
        'person'
        'pottedplant'
        'sheep'
        'sofa'
        'train'
        'tvmonitor'};

所以你懂得，错误就在类别上，把VOCopts.classes改成自己需要是别的类别。

如：VOCopts.classes={'airplane'}

贴个错误图片看看：