Type does not have null as a proper value

Use the [<AllowNullLiteral>] attribute:

[<AllowNullLiteral>]
type public MyClass(reasonForLiving:string) =
    member x.ReasonForLiving with get() = reasonForLiving

By default, F# types do not allow null (thank heavens!). This attribute is useful for interop with other .NET languages and allows assignment/comparison with null.