Difference of two convex functions using cvx

suppose c & d is any convex function of (X,y). So,the problem is
min_{X,y} c-d. How it is defined using cvx. when I perform this I get error as:

??? Error using ==> cvx.plus at 83

Disciplined convex programming error:

Illegal operation: {convex} - {convex}

Error in ==> cvx.minus at 21

z = plus( x, y, true, cheat );

CVX does not allow this, because the result is very rarely convex.

Is any way to convert it into convex.