Narxnet, Error using struct, Conversion to struct from double is not possible.

In the Matlab UserGuide for Neural Networks Toolbox R2012a (page 3-25), there is an example of a Dynamics Network with the following code:

load magdata
[u,us] = mapminmax(u);
[y,ys] = mapminmax(y);
y = con2seq(y);
u = con2seq(u);
d1 = [1:2];
d2 = [1:2];
narx_net = narxnet(d1,d2,10);
narx_net.divideFcn = '';
narx_net.trainParam.min_grad = 1e-10;
[p,Pi,Ai,t] = preparets(narx_net,u,{},y);
narx_net = train(narx_net,p,t,Pi);
yp = sim(narx_net,p,Pi);
e = cell2mat(yp)-cell2mat(t);
plot(e);

But when I use the code above I get the following error message:

Error using struct Conversion to struct from double is not possible.

However when I use:

narx_net = narxnet(1:2,1:2,10);

Instead of:

d1 = [1:2];
d2 = [1:2];
narx_net = narxnet(d1,d2,10);

The program runs okay.

Then when I try to run the same code but with my own input and output data. So I define u and y myself, I got the same error message again.

Even more strange when I then try to run the original code (see below) then I also get the error message, while previous the code run fine??? When I restart Matlab, the code below works fine again???

load magdata
[u,us] = mapminmax(u);
[y,ys] = mapminmax(y);
y = con2seq(y);
u = con2seq(u);
narx_net = narxnet(1:2,1:2,10); 
narx_net.divideFcn = '';
narx_net.trainParam.min_grad = 1e-10;
[p,Pi,Ai,t] = preparets(narx_net,u,{},y);
narx_net = train(narx_net,p,t,Pi);
yp = sim(narx_net,p,Pi);
e = cell2mat(yp)-cell2mat(t);
plot(e);

Hopefully someone can help me with this mystery. Many thanks