No out of memory

I only allocate 16 ints, but can set the value of the 10000 int. Why does this program runs fine without memory error ?

#nowarn "9"

open System
open System.Runtime.InteropServices
open Microsoft.FSharp.NativeInterop

let si:int=sizeof<int>
let nulpointer:nativeptr<int> = NativePtr.ofNativeInt<int> 0

type MyClass() =
    do printfn "Create"

    let mutable ptypedback:nativeptr<int>=nulpointer

    member this.puntyped: nativeint = Marshal.AllocHGlobal (si*16)

    member this.ptyped
        with get (): nativeptr<int> = ptypedback
        and set (value: nativeptr<int>) = ptypedback <- value

    member this.setptr = this.ptyped 
                           <- NativePtr.ofNativeInt<int> this.puntyped

    interface IDisposable with
        member this.Dispose() =
            Marshal.FreeHGlobal this.puntyped
            printfn "Destroy"

let myfun  =
    use c = new MyClass()
    let set10000 = NativePtr.set c.ptyped 10000
    set10000 123
    let get10000 =NativePtr.get c.ptyped 10000
    printfn "Value at index 10000 : %A " get10000