Why does kron(D,I), with D and I both sized (nxn), not create a matrix sized (n^2 x n^2)?

I am trying to run a code that involves operating on the elements of a matrix made by a kronecker sum operation, but I meet a sub2ind error. I have found that the problem is that MATLAB doesn't generate the matrix size it is supposed to generate after one kind of kronecker product operation in particular:

x = sym('x', [1 15], 'real')

x = 

I = eye(5)

I = 5×5

1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1

D = diag(-x(1) -x(2) -x(3) -x(4) -x(5))

D = 

disp(kron(I,D))

Which causes an error in my code that involves the following:

DsumD = kron(D,I) + kron(I,D)

DsumD = 

zeta = sym(zeros(1,25));
for i = 1:25
    zeta(1,i) = -DsumD(i,i);
end

Error using sub2ind (line 43)
Out of range subscript.

Error in sym/subsref (line 992)
R_tilde = sub2ind(size(L), Idx.subs{:});

zetatopi = sym(zeros(1,25));
for j = 1:25
    zetatopi(1,i) = log(1+zeta(1,i)*x(15))/x(15);
end
DsumDtopi = diag(-zetatopi)

Does anyone know how I can make MATLAB do the kronecker operations properly?

Answer 1

[Paul]

Because D is not 5 x 5

x = sym('x', [1 15], 'real');
D = diag(-x(1) -x(2) -x(3) -x(4) -x(5))

D = 

size(D)

ans = 1×2

1 1

Perhaps you meant

D = diag(x(1:5))

D = 

or

D = diag([-x(1) -x(2) -x(3) -x(4) -x(5)])

D = 

[Rex Mantiri]

Wow that's a pretty silly mistake on my part. Thanks for the help Paul!

[Rex Mantiri]

@Paul I have another question thread for code used in the same research. Here's the link: https://www.mathworks.com/matlabcentral/answers/1608610-how-do-i-estimate-15-parameters-with-the-least-square-method-for-a-non-linear-objective-function-ma. Any input you have on the matter is greatly appreciated.