[R] constrOptim and function with additional parameters?

[Marlene Mueller]

How can I use a function with some additional input parameters
in constrOptim? For example, something like

fr <- function(x,a) {   ## Rosenbrock Banana function
  x1 <- x[1]
  x2 <- x[2]
  a * (x2 - x1 * x1)^2 + (1 - x1)^2
}

where the optimum is to be found w.r.t. x. Calling
optim(c(-1.2,1), fr, NULL, a=100) works as expected, but I fail 
to provide the a=100 in the constrained case:

>  constrOptim(c(-1.2,0.9), fr, NULL, ui=rbind(c(-1,0),c(0,-1)),
ci=c(-1,-1),a=100)
Error in f(theta) : Argument "a" is missing, with no default

Is this a bug or is there a different solution that I miss here?

TIA, Marlene