sandwich matrix in gamm

Hello, 

I need to calculate a sandwich covariance matrix in gamm. The package
sandwich does not work because:
Error in UseMethod("vcov") : no applicable method for 'vcov' applied to an
object of class "c('gamm', 'list')"

Does anyone knows how to overcome this problem? Thanks in advance.

Regards,
Isabel Proen?a
Dep. of Mathematics
ISEG ? UTL (Technical University)