How can I test if a sequence is empty in F#?

Consider this F# code which computes a factor of a number:

let n = 340339004337I

// A sequence of all factors:
let factors = seq { 1I .. n / 2I} |> Seq.filter (fun x -> n % x = 0I) 

// Pull off the first factor from the sequence:
let factor = 
    if factors = seq [] then
        n
    else
        factors |> Seq.nth 0

In other words, if factors is empty, then return n. Otherwise, pull off the first element from factors. The goal is to account for all factors between 1 and (n/2), and n itself since 1 and n are always factors of n.

The factors = seq [] test isn't working. I arrived at this syntax by looking at this:

> seq {1 .. 100} |> Seq.filter (fun x -> false) ;;
val it : seq<int> = seq []

However, I don't think seq [] is actually an empty sequence:

> Seq.empty = seq [] ;;
val it : bool = false

How can I test if a sequence is empty?