Help understanding environments

I have a collection of .RData files that have result objects that I would like to combine.  Specifically, skatCohort objects from the skatMeta package, but I've run into a similar issue with simple data.frames also.

If I run something like

FILES <- list.files(path="/path/to/my/results", pattern=".RData$", 
full.names=TRUE) combined <- NULL
   for (FILE in FILES) {
     load(FILE)
     if (!is.null(combined)) {
       combined <- c(combined, res)
     } else {
       combined <- res
     }
   }

I get all my objects combined.  However, if I wrap this into a 
function I get the following error

c_objects <- function(FILES) {
   combined <- NULL
   for (FILE in FILES) {
     load(FILE)
     if (!is.null(combined)) {
       combined <- c(combined, res)
     } else {
       combined <- res
     }
   }
   return(combined)
}

combined_results <- c_objects(FILES)
Error in eval(expr, envir, enclos) : object 'combined' not found

How should I write this function such that it can find "combined".   I've tried reading the help on envirnaments, and the exisits function but I haven't been able to figure this out.  Are there any other resources to read up on this?

Thanks in advance,

-Brian