Error in DAGNetwork/predict

Calling the matlab function predict gives the following error:

Output argument "varargout{2}" (and maybe others) not assigned during call to "DAGNetwork/predict".

Here is my function. This is from the Object Detection Using YOLO v3 tutorial.

function YPredCell = yolov3Predict(net,XTrain,networkOutputs,anchorBoxMask)
% Predict the output of network and extract the confidence, x, y,
% width, height, and class.
YPredictions = cell(size(networkOutputs));
[YPredictions{:}] = predict(net, XTrain);
YPredCell = extractPredictions(YPredictions, anchorBoxMask);

% Apply activation to the predicted cell array.
YPredCell = applyActivations(YPredCell);
end

Why am I receiving this error?