Problems with running diagnostic plot for a lmer object

Dear all,

I have recently learn how to run logistic regression with lmer. But I
have problems with running a diagnostic plot. I have tried plot(mod),
but get this message:
Error in object$coefficients : $ operator not defined for this S4
class. Tried to search for it, but without success. What means this?

Is there other ways to do a diagnostic plot for an lmer object?

Regards Cotter