LiveChart throws exception (despite working ostensibly ok)

When run, the following code silently throws an exception at the line:

```
let chartControl = new ChartControl(chart),
```

despite ostensibly running just fine. The message in the output window is:
"A first chance exception of type 'System.InvalidOperationException' occurred in FSharp.Core.dll"

```
open System
open System.Drawing
open System.Reactive.Linq
open System.Threading
open System.Windows.Forms
open FSharp.Charting
open FSharp.Charting.ChartTypes

// Learn more about F# at http://fsharp.net
// See the 'F# Tutorial' project for more help.

[]
let main argv =
printfn "%A" argv

let form = new Form()
form.Size <- new Size(800, 600)

let step =
let gen = new Random()
(fun () -> 2.0 * (gen.NextDouble() - 0.5))

let randomWalk =
Observable
.Interval(TimeSpan.FromMilliseconds 200.0)
.ObserveOn SynchronizationContext.Current
|> Observable.scan (fun (x, y) _ -> (x + 1.0, y + step())) (0.0, 0.0)

let chart = LiveChart.FastLineIncremental randomWalk

let chartControl = new ChartControl(chart)
chartControl.Dock <- DockStyle.Fill

form.Controls.Add chartControl

Application.Run(form)
0 // return an integer exit code
```

Am I doing something that I shouldn't be doing?