Private constructor in sum type crashes `sprintf "%A"`?

sample.fs

module MyModule

open Xunit
open FsUnit.Xunit

type Sample = Sample of int
    with override x.ToString() = sprintf "%A" x
[<Fact>]
let ``sample to string`` ()=
    let actual = Sample 1
    actual.ToString() // all good

type Sample2 = private Sample2 of int
    with override x.ToString() = sprintf "%A" x
[<Fact>]
let ``sample2 to string`` ()=
    let actual = Sample2 1
    actual.ToString() // Stack overflow?!??!