Disciplined convex programming error:CVX_real affine.*convex

 Chi1_bar, a1, b1, X1,Y1 are constants and tau(1,1), t(1,1) and Vo are cvx variable.
Vo has order(1:N+1,1:N+1).

Giving error:
Disciplined convex programming error:
Cannot perform the operation: {real affine} .* {convex}

I believe log((X1*t(1,1)*inv_pos(tau(1,1)+Y1*t(1,1)))-1) is neither concave nor convex. if you have proof otherwise, please show it.