trace in CVX package

Question

I am using trace in my CVX (MATLAB) code. Sometimes it is working fine, but sometimes it is showing some errors. Error message is as follows-

"trace" previously appeared to be used as a function or command, conflicting with its use here as the name of a variable. A possible cause of this error is that you forgot to initialize the variable, or you have initialized it implicitly using load or eval.

I searched in CVX website and I found this-

trace(Z) is valid only if the elements along the diagonal have the same curvature.

I could not make sense out of it. Looking forward for any generous suggestion.

I excerpt a part of my MATLAB code below to clearly state my problem.

The program where it is working properly-

%% Calling CVX Package
    G = zeros((M+1)*d,(M+1)*d);
    for i = 0:M
        G(i*d+(1:d),i*d+(1:d)) = eye(d);
    end
    cvx_precision best;
    cvx_begin
    variable G((M+1)*d,(M+1)*d) semidefinite           % Defining variables
    minimize(trace(C*G))                               % Objective function
    subject to
    % Constraints
    for i = 0:M
        G(i*d+(1:d),i*d+(1:d)) == eye(d);
    end
    cvx_end

The program where it is giving errors-

%% Calling CVX Package
Q = zeros(N,N);
    Mij = zeros(N,N);
    cvx_precision best;
    cvx_begin
    variable Q(N,N) semidefinite                       % Defining variables
    minimize(trace(Q))                                 % Objective function
    subject to
    % Constraints
    for i = 1:N-1
        for j = i:N
            if E(i,j) ~= 0
                Mij = Mij-Mij;
                Mij(i,j) = -1;
                Mij(j,i) = -1;
                Mij(i,i) = 1;
                Mij(j,j) = 1;
                trace(Mij*Q) = E(i,j);
            end
        end
    end