optim function argument missing

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This is my code. The kum.loglik function returns negative loglikelihood and takes two arguments a and b. I need to find a and b that minimize this function using optim function. (n1,n2,n3 is pre-specified and passed to optim function.

kum.loglik = function(a, b, n1, n2, n3) {
  loglik = n1*log(b*beta(1+2/a,b)) + n2 * log(b*beta(1+2/a,b)-2*b*beta(1+1/a,b)+1) +
    n3 * log(b*beta(1+1/a,b)-b*beta(1+2/a,b))
  return(-loglik)
}
optim(par=c(1,1), kum.loglik, method="L-BFGS-B",
      n1=n1, n2=n2, n3=n3,
      control=list(ndeps=c(5e-4,5e-4)))

This code should work well but it gives error message 

Error in b * beta(1 + 2/a, b) : 'b' is missing

What is wrong in this code?

r optimization
2 Answers

I always use the following, it gives you the best results

p0 <- c(a,b) #example vector of starting values
m <- optim(p0, loglik, method="BFGS", control=list(fnscale=-1, trace=10),
hessian=TRUE, x=data.frame)
#for table of results
rbind(m$par, sqrt(diag(solve(-m$hessian))))
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