[R] AIC for heckit

[Werner Wernersen]

Hi,

I have used the heckit function in micEcon. Now I
would like to evaluate the fit of the probit part of
the model but when I enter
AIC(sk$probit)

I get this error
Error in logLik(object) : no applicable method for
"logLik"

How can I then get the AIC for this model?

Side question:  If you know - from the top of your
head - some link to readings dealing with evaluating
the appropriateness of the model and the fit with help
of an example, please let me know!

Thanks a million,
  Werner