One or more output arguments not assigned

Question

I want to implement a convex-relaxation for the E-optimality design criterion, that is: By defining the optimization problem would be

I have implemented in Matlab using this function

function [d] = e_optimality(Uk, s, sigma)
%% E-OPTIMALITY DESIGN CRITERION, CONVEX RELAXATION
%   
%       OUTPUT: sampling_set= vector with ordered sampling
%       set
%
%       INPUT: Uk= NxK Laplacian eigenvector in the selected band
%              s= number of samples
%              sigma: entry noise matrix

N= size(Uk,1);
Sigma= diag(sigma*ones(N,1))
Sigma_inv= inv(Sigma);


cvx_begin
variable d(N)
D= diag(d)
B= Uk'*Sigma_inv* D*Uk;
minimize -lambda_min(B)
subject to
           d*ones(N,1)==s;
           0<=d<=1;
cvx_end

Unfortunately, when launghing the function I have the following error:

One or more output arguments not assigned during call to "varargout".

Error in minimize (line 14)
    x = evalin( 'caller', sprintf( '%s ', varargin{:} ) );

Error in e_optimality (line 20)
    minimize -lambda_min(B)

It seems to be the minimize -lambda_min in CVX causing the problem. What could it be? Thank you