CVX DCP Rule Error


I would like to know why I am receiving the following error in CVX:

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

after typing something like (A*x+b)inv_pos(cx+d)

I figure out that I need inv_pos to represent 1 over the polynomial and I know the first parentheses is affine and although it is convex but according to CVX it is not considered to be convex , so how can I multiply (A*x+b) by inv_pos ?