R help - Jan 2016 - pgmm error message, Error in terms.default(formula) : no terms component nor attribute

Dear

I try to use pgmm to estimate dynamic model for panel data. I got the message
below. It is appreciate that you could help on this.
data<-read.csv("G:/MMU/Research/Urbanlisation/paneldata.csv",sep=",",
header=TRUE) library(plm) library(Formula) urban<-pdata.frame(data,
index=c("id","year_begin")) y<-urban[,7] x1<-urban[,8]
x2<-urban[,9] x3<-urban[,10] x4<-urban[,11]
z1<-pgmm(formula=y~lag(y,0:1)+x2+x3+x4,data=urban, effect =
"twoways", model = "twosteps") Error in
terms.default(formula) : no terms component nor attribute

Xin
-------
Dr.Xin Shi
Reader in Applied Statistics
Director of DBA Programme
Room 609
Department of Marketing, Operations and Digital Business
Business School
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Manchester Metropolitan University
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E-mail: X.Shi at mmu.ac.uk<mailto:X.Shi at mmu.ac.uk>
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