Why the derivation calculation can't be done in the function block?

Attached pls find my mathcad worksheet. In this worksheet, I define a function Vout. First I calculate the R1 derivative of the function (step 1), it works. Then I use a function block to do the sensitivity analysis of R1 for the function Vout (step 2), but it report that "Can't calculate the expression". Why the step 1 can work, but step 2 can't?

Thanks,

Simon