coping with a warning in if()

Hi,

I'm sure this one is very easy....

I am trying to write a function where one of its arguments has two posible (strings) values, 
defaulting to one of them if none is specified. My problem is that when evaluating the function 
the following warning is produced:

"the condition has length > 1 and only the first element will be used"

I've read the help page of if() but I don't get it because when evaluating step by step the 
warning is not produced.

Omitting the details of my function, below is an example of this 

## Function
fun <- function( n, result = c("simple", "complete") )
{
  if ( is.null(result) ) 
     result <-  "simple" 
  if ( !(result %in% c("simple", "s", "complete", "c")) ) {
      stop("specify type of result 'simple' or 'complete'")
 } else {
  res <- rnorm(n)
  res
 }
}