Arxstruc with multiple experiment

Hello,

I am running an identification experiment with a set of experiment. I am using a total of 4 experiments with input output data.

I am using the command arxstruc to find the best structure for my model.

for i = 1:4
   z{i} = iddata(output{i},input{i}) % there are 3 inputs
   z{i} = detrend(z{i},1);
end
z = merge(z{:});
zi = getexp(zd,[1 2]);
zv = getexp(zd,4);
argsNN = [{1:4} repmat({1:4},1,3) repmat({1:3},1,3)];
NN = struc(argsNN{:});
V = arxstruc(zi, zv, NN);

Now when I am using arxstruc I get the error:

Error using arxstruc (line 132)
In the "arxstruc(ZE,ZV,NN)" command, ZE and ZV must have the same number of inputs, outputs and data experiments.

Indeed, if I use instead for validation 2 experiments:

zv = getexp(zd,[3 4])

Then it works but I don't see why that should be and I don't want to use the same number of experiments for validation and estimation anyway. Any ideas,

Thanks in advance