Error using cvx/min (line 208) Disciplined convex programming error: Invalid computation: min( {convex} )

Question

my code

cvx_begin
    variable Q_new(N,2)
    gama0 = P * db2pow(row0) / sigma2;
%     R = zeros (K,1) ;
    expression R(K,1);
    expression r;
for i = 1 :1: K