[R] strange behaviour of negative binomial

[dax42]

Dear list,

I ran into a strange behaviour of the pnbinom function - or maybe I 
just made a stupid mistake.
First thing is that pnbinom seems to be very slow. The other - more 
interesting one - is that I get two different curves when I plot the 
estimated density and the density given by pnbinom. Shouldn't it be the 
same?
This is only the case, I think, if I use the parameter size = 1. I just 
tried it for size = 2 and the result seemed correct..

Anyways, this is what I tried:

#parameters
proba<-0.001039302;
s<-1;

#random numbers with this distribution
dom<-rnbinom(10000,s,proba);

#first plot
plot(ecdf(dom),do.points=FALSE);

#second plot
x<-seq(0,8700,0.5);
lines(x,pnbinom(x,s,proba),col="red");