MergeSort algorithm in F#

MergeSort.fs

let mergeSort xs =
    let wrap x = [x]
    let rec merge = function
            | x::xs, y::ys -> if x < y then x :: merge (xs, y::ys) else y :: merge (x::xs, ys)
            | l, [] -> l
            | [], m -> m
    let rec mergePairs = function
            | (l::m::ns) -> merge (l, m) :: mergePairs ns
            | ls -> ls
    let rec mergeAll = function
            | [] -> []
            | [l] -> l
            | ls -> mergeAll (mergePairs ls)
    mergeAll (List.map wrap xs);;
 
let genRandomNumbers count =
    let rnd = System.Random()
    List.init count (fun _ -> rnd.Next (10000));;
 
printf "%A\n" <| mergeSort (genRandomNumbers  80000);; // OK
printf "%A\n" <| mergeSort (genRandomNumbers 100000);; // stack overflow