# I just replace wages below 64 with zero to obtain a data that works.
library(micEcon)
lnu$wage <- replace(lnu$wage, lnu$wage<64, 0)
heckit(log(wage) ~ female+school+exp+I(exp2), I(wage>0) ~ female+school+exp+I(exp2)+public, data=lnu, print.level=1)