NARX Neural Network test on new test set?

After training NAREX net.I want to test the model on independent test set "testX'".But it returns me an error.I am testing the model by forecastLoad = sim(net, testX')'.

%Create training set

trainInd = data.NumDate < datenum('2008-01-01');
trainX = X(trainInd,:);
trainY = data.SYSLoad(trainInd);

% Create test set to test later
testInd = data.NumDate >= datenum('2008-01-01');
testX = X(testInd,:);
testY = data.SYSLoad(testInd);
testDates = dates(testInd);

X = tonndata(trainX,true,false);
T = tonndata(trainY,true,false);

% Choose a Training Function

trainFcn = 'trainlm';  % Levenberg-Marquardt backpropagation.

% Create a Nonlinear Autoregressive Network with External Input
inputDelays = 1:25;
feedbackDelays = 1:25;
hiddenLayerSize = 10;
net = narxnet(inputDelays,feedbackDelays,hiddenLayerSize,'open',trainFcn);

% Prepare the Data for Training and Simulation

[x,xi,ai,t] = preparets(net,X,{},T);

% Setup Division of Data for Training, Validation, Testing
net.divideParam.trainRatio = 70/100;
net.divideParam.valRatio = 15/100;
net.divideParam.testRatio = 15/100;

% Train the Network
[net,tr] = train(net,x,t,xi,ai);

% Test the Network
y = net(x,xi,ai);
e = gsubtract(t,y);
performance = perform(net,t,y)

% View the Network
view(net)

%%Forecast using Neural Network Model
% Once the model is built, perform a forecast on the independent test set. 

load Data\testSet
forecastLoad = sim(net, testX')';

[Greg Heath]

Whenever you post code, it is wise to use MATLAB example data obtained from the commands

 help nndatasets

and

 doc nndatasets.

Hope this helps.

Greg