R help - Apr 2009 - summary.plm error

Dear plm Package users, 
 
I use the plm package  a lot but I have not updated it for some times.
Now I realized the following difficulty with the summary.plm function
(demonstrated with the example from the ?plm documentation).
 
 
 
library(plm)
 
data("Produc", package="Ecdat")
 
estimation_method<-"within"
estimation_effect<-"individual"
 
     zz <- plm(log(gsp)~log(pcap)+log(pc)+log(emp)+unemp, data=Produc,
index=c("state","year")
     , method=estimation_method
     , effect=estimation_effect
    )
     summary(zz)
Error: object of type 'symbol' is not subsettable
 

But if I run 
 
     zz <- plm(log(gsp)~log(pcap)+log(pc)+log(emp)+unemp, data=Produc,
index=c("state","year")
     , method="within"
     , effect="individual"
    )
     summary(zz)
 
the summary function works fine.
 

Is it wrong (or bad practice) to define the function's parameters as I
did in the upper example? If yes, how should it be done correctly such
that the summary function works?
 
I am running R 2.8.1 wie plm versin 1.1-1 on a Windows XP workstation.
 
Thank you for your advice, Ulrich Morawetz
 

 

	[[alternative HTML version deleted]]