problems with inverse mills ratio when using heckman.

[yang zx]

I have a question.when I am dealing with the classical sample selection problem,I will do

probit z $zlist
predict xb,xb
gen mylambda = normalden(xb) / normal(xb) if z=1
replace mylambda = -normalden(xb) / normal(-xb) if z=0
reg wage $xlist mylambda if z==1
predict predictwage,xb

But when I use the heckman command, and save the lambda

heckman wage $xlist,select(z=$zlist) twostep m(lambda)


It seems that lambda= normalden(xb) / normal(xb) for everyone.
And this problem will associate with the problem of predicted wages for the unemployed.

Which one is right?

I am looking forward for your help