NaNS Error Message

Hi,
I'm using R to do a series of calculation and I have gotten several
warnings that say "NaNS produced". Whatever I could read on line gives me
an idea that this warning is produced when the number is use is a negative
log or otherwise mathematically problematic.
I'm getting this error while using factorial() and gamma () on strictly
positive numbers (always greater than zero).
Here is a snippet of my code:
k goes from 1 to a positive limit.
S is always 100.
Only positive values of n are allowed into the loop.

if(n>0)
{
while(k<=lim)
      {

        sn<-(S-n);snfact<-gamma(sn);
        sn2<-(2-n+S);sn2gam<-gamma(sn2);

        num<-(ngam*sn2gam);


        nk2<-(2+k-(2*n)+S);
        nk2gam<-gamma(nk2);
        den<-(k*nk2gam);
        prob<-(num/den);

        sum<-(as.numeric((k*prob))+sum);

        k<-k+1;
      }
}

The error message  received for every instance of this loop is :
In gamma(sn) : NaNs produced
In gamma(sn2) : NaNs produced
In gamma(nk2) : NaNs produced