Index exceeds array bound error when using MATLAB deep learning tutorial code

I am using the deep learning tutorial series code found at https://uk.mathworks.com/matlabcentral/fileexchange/62990-deep-learning-tutorial-series

The code filename is Demo_TrainingFromScratch

The problem that i am having is that when i run the test data i get an error saying Index exceeds array bound. The error appears below this code:

labels = classify(net, imds_test);

ii = randi(4000); im = imread(imds_test.Files{ii});

The only thing i have changed are the images used to train the machine and the name and location of the folders and files. Please note the error only comes during the test data phase.

Thank you