when i run an cvx code,the window always show me this error

Error using .* (line 173)

Disciplined convex programming error:

Cannot perform the operation: {real affine} ./ {real

affine}Error in ./ (line 19)

z = times( x, y, ‘./’ );Error in * (line 36)

z = feval( oper, x, y );Error in / (line 15)

z = mtimes( x, y, ‘rdivide’ );Error in FIRSTphase (line 69)

H1_A=2*(1-xa)/xa

Because i have checked that (1-xa)/xa is a convex,but i don’t know why the cvx doesn’t want to accept my code,should i write it as the other way? or the denominator can’t be variable when the Numerator is the variable too?