RCNN Object Detection Example has [ No public property 'NumObservations' for class 'ParallelRegionReader' ]

I'm trying to learn the RCNN Object detection by Matlab Example (My version is R2017b, and I follew the website instructions step by step.https://ww2.mathworks.cn/help/vision/examples/object-detection-using-deep-learning.html )

However, when I change the do Tarining to true and run, I get the following errors.

I don't know what is the 'NumObservations' and the tutorial also doesn't mention this.

Do someone know how to fix this problem ?

Thanks

% To save time while running this example, a pre-trained network is loaded
% from disk. If you wish to train the network yourself, set the
% |doTraining| variable shown below to true.
%
% Note that a CUDA-capable NVIDIA(TM) GPU with compute capability 3.0 or
% higher is highly recommeded for training.

% A trained detector is loaded from disk to save time when running the
% example. Set this flag to true to train the detector.
doTraining = true;

if doTraining
    
    % Set training options
    options = trainingOptions('sgdm', ...
        'MiniBatchSize', 128, ...
        'InitialLearnRate', 1e-3, ...
        'LearnRateSchedule', 'piecewise', ...
        'LearnRateDropFactor', 0.1, ...
        'LearnRateDropPeriod', 100, ...
        'MaxEpochs', 100, ...
        'Verbose', true,...
        'Plots','training-progress');
    
    % Train an R-CNN object detector. This will take several minutes.    
    rcnn = trainRCNNObjectDetector(stopSigns, cifar10Net, options, ...
    'NegativeOverlapRange', [0 0.3], 'PositiveOverlapRange',[0.5 1])
else
    % Load pre-trained network for the example.
    load('rcnnStopSigns.mat','rcnn')       
end

Step 1 of 3: Extracting region proposals from 41 training images...done.

Step 2 of 3: Training a neural network to classify objects in training data...

Error using trainNetwork (line 140)
No public property 'NumObservations' for class 'ParallelRegionReader'.

Error in rcnnObjectDetector.train (line 222)
            detector.Network = trainNetwork(dispatcher, layers, opts);

Error in trainRCNNObjectDetector (line 197)
detector = rcnnObjectDetector.train(trainingData, layers, options, params);

Error in DeepLearningRCNNObjectDetectionExample (line 378)
    rcnn = trainRCNNObjectDetector(stopSigns, cifar10Net, options, ...

Caused by:
    No public property 'NumObservations' for class 'ParallelRegionReader'.