Errors While Trying To Extract Images Features using Pretrained AlexNet Model

hello I have 273 grayscale Images (Drone) and I want to Apply Faster R-CNN model to Classify Objects in images, but in first I used A Pretrained AlexNet Model for Increase Speed of my project, But in while trying To Extract Images Features using Pretrained AlexNet Model and writing below codes from Mathworks site (www.mathworks.com/help/deeplearning/ref/alexnet.html - Extract Image Features )

layer = 'fc7';
featuresTrain = activations(net,augimdsTrain,layer,'OutputAs','rows');
featuresTest = activations(net,augimdsTest,layer,'OutputAs','rows');

Matlab showed Below errors:

Error using nnet.internal.cnn.validateMiniBatchDatastore>checkPartitionByIndex (line 90)

Incorrectly defined MiniBatchable Datastore. Error in partitionByIndex method of F:\Program

Files\MATLAB\R2018a\toolbox\nnet\cnn\+nnet\+internal\+cnn\ImageDatastoreMiniBatchDatastore.m at line 59:

Undefined function 'categories' for input arguments of type 'double'.

Error in nnet.internal.cnn.validateMiniBatchDatastore (line 16)

checkPartitionByIndex(ds);

Error in nnet.internal.cnn.DataDispatcherFactory.createDataDispatcher (line 67)

nnet.internal.cnn.validateMiniBatchDatastore(inputs);

Error in SeriesNetwork>iDataDispatcher (line 1112)

dispatcher = nnet.internal.cnn.DataDispatcherFactory.createDataDispatcher( ...

Error in SeriesNetwork/activations (line 790)

dispatcher = iDataDispatcher( X, miniBatchSize, precision, ...

Error in myprj (line 24)

featuresTrain = activations(net,augimdsTrain,layer,'OutputAs','rows');

Can AnyOne Help Me?

[Mark Sherstan]

I remember getting a similar error when I did not resize my images to 227-by-227. Here is some code that I wrote that you could try your data on here and my resize and images processing function here.