optim and "the function should not" advice

Dear R users,

I'm using optim to optimize a pretty complicated function. This function takes the parameter vector "theta" and within its body I use instructions like

sigma<-theta[a:b]; computations with sigma...
out<-c()
for (i in 1:d){
a<-theta[(3*d+i):c]
out[i]<-evaluation of an expression involving 'a' (I use symbolic differentiation)
}

Unfortunately for certain problems 'optim' returns a parameter vector which didn't move at all from the initial parameters, and the output says that although the function has been evaluated a high number of times, the gradient (which I fed the function with) has been evaluated only one time. I used the BFGS method.

By chance I looked at the help and I read "The parameter vector passed to fn has special semantics and may be shared between calls: the function should not change or copy it" . Could the instructions above be the cause of the failure? If so, how to deal with symbolic differentation?

Thanks in advance,
Gildas