R help - Apr 2008 - nls plinear formula

I want to fit a nonlinear model of the form:
Y=A+B*X1+C*X2+log(X3/(X3+D))
I think that the best way is to use the plinear algorithm, but I don't 
know how to specify the formula in the nls function.
I've tried:
Y~cbind(rep(1,times=length(Y)),X1,X2,log(X3/(X3+D)))
But this fits the model:
Y=A+B*X1+C*X2+D1*log(X3/(X3+D))
How can I specify the formula correctly?
Thanks,
M