[RsR] SES

Hi
I was trying to estimate simultaneous equation system in R using systemfit.
I used the following commands

library(systemfit)
data(Kmenta)
attach(Kmenta)

>eqDemand<-consump~price+income

eqSupply<-consump~price+farmprice+trend
fitsur<-systemfit("SUR",list(demand=eqDemand, supply=eqSupply))

and got the following error messege

Error in systemfit("SUR", list(demand = eqDemand, supply = eqSupply)) :
argument 'formula' must be an object of class 'formula' or a list of objects
of class 'formula'

Can anyone tell me how to overcome this problem?

regards

Anwesha