library(bbmle)
d <- data.frame(x=0:10,
                y=c(26, 17, 13, 12, 20, 5, 9, 8, 5, 4, 8))

maxit <- 1000
mle2(y~dpois(lambda=exp(lymax)/(1+x/exp(lhalf))),
   start=list(lymax=0,lhalf=0),
   data=d,
     control=list(maxit=maxit),
   parameters=list(lymax~1,lhalf~1))