Class "coef.mer" into a data.frame?

Hello,

Could somebody please tell me what am I doing wrong in following?

I try extract coefficients (using arm-package) from the lmer  
frunction, but I get the
following warning:
a<-data.frame(coef(res))
Error in as.data.frame.default(x[[i]], optional = TRUE,  
stringsAsFactors = stringsAsFactors) :
   cannot coerce class "coef.mer" into a data.fram

I think I have done it before the same way and it has worked, but not  
any more..

Jukka