help with GAM

Hi

I'm trying to do a GAM analysis and have the following codes entered
into R (density is = sample number, alive are the successes)

density<-as.real(density)

y<-cbind(alive,density-alive)

library(mgcv)

m1<-gam(y~s(density),binomial)

at which point I get the following error message

Error in smooth.construct.tp.smooth.spec(object, dk$data, dk$knots) :
A term has fewer unique covariate combinations than specified maximum
degrees of freedom

What am I doing wrong?  Please help!

Thanks!