Error or mismatch in the specified ODE file when I use the Nonlinear Grey-Box Models

Hi, I am trying to estimate parameters in my nonlinear grey box model. The model file was prepared:

function [dx, y] = myode(t, x, u, para, varargin)

where the 'para' was a parameter vector including 15 parameters. Then as needed, I used other variables to represent the parameters:

mtx=zeros(15,15);

for i=1:15

     mtx(i,i)= para(i);

........

And then the idnlgrey object was defined. However, when I used 'compare' and 'pem', there was an error:

Error using idnlgrey/isvalid (line 166) Error or mismatch in the specified ODE file "myode". The error message is: "Attempted to access para(2); index out of bounds because numel(para)=1."

But, when I used ode solver to solve the ode with assumed parameters, it did work!

What's the reason and how can I manage it?

Any tips would be much appreciated! Thank you!