Holt Winters Error

Dear Group,

I have come across with the following error while
running HoltWinters().

optim(c(0.3, 0.1, 0.1), error, method = "L-BFGS-B", 
lower = c(0, 0, 0), upper = c(1, 1, 1))

It only happens to multiplicative seasonality but not
additive.  I read the program but still couldn't
understand it.  

Thanks!