Maximum likelihood estimation-error

I am trying to use MLE to estimate a model parameters, and also calculate the covariance matrix. I get zero for the variance of one of the parameters. Can this be a correct answer?
I appreciate any help on this.

That is (almost) impossible. Did you check the mode_check-plots?

Hi, many thanks for reply. What do you mean by mode_check-plot. Sorry if the answer is obvious. Could you advise please?

By the way, let me tell you that I am developing a Matlab code based on MLE.

In Dynare’s estimation-command, there is the mode_check-option that provides slices through the likelihood at the mode.

Many thanks for your help.