Grey-box model parameter standard deviation (nlgreyest)

Hello,

Under which conditions you can't get the standard deviation of the model parameters using nlgreyest?

I tried with a 1-parameter simple model and it works. I get the parameter and its deviations.

When i use a more complex model (19 parameters with some of them correlated), even though i'm using the exactly same procedure, I get the parameters right but no deviation (vector is empty).

Thanks!