Foq cannot explicitly return an F# type as null

FoqReturns.fs

module ``Foq cannot explicitly return an F# type as null``

open System
open Foq

// Discriminated Unions

type TimeInForce = GoodTillCancel | GoodTillDate of DateTime

type IOrder =
    abstract LimitPrice : decimal
    abstract Quantity : int
    abstract TimeInForce : TimeInForce

let ``can setup member to return DU as value`` =
    Mock<IOrder>().Setup(fun x -> <@ x.TimeInForce @>).Returns(GoodTillCancel)

let ``cannot setup member to return DU as null`` = 
    // Error: No overload match for method 'Returns'
    Mock<IOrder>().Setup(fun x -> <@ x.TimeInForce @>).Returns(null) 

let ``cannot mock with member returning DU as null`` =
    // Error: The type 'TimeInForce' does not have 'null' as a proper value 
    Mock<IOrder>.With(fun x -> <@ x.TimeInForce --> null @>)

// Record types

type Counterparty = { Name : string; Company : string }

type ITrade =
    abstract Price : decimal
    abstract Quantity : int
    abstract Initiator : Counterparty
    abstract Aggressor : Counterparty

let ``can setup member to return record as value`` =
    let initiator = { Name = "Name"; Company = "Company" }
    Mock<ITrade>().Setup(fun x -> <@ x.Initiator @>).Returns(initiator)

let ``cannot setup member to return record as null`` =
    // Error: No overload match for method 'Returns' 
    Mock<ITrade>().Setup(fun x -> <@ x.Aggressor @>).Returns(null)

let ``cannot mock with member returning record as null`` =
    // Error: The type 'Counterparty' does not have 'null' as a proper value
    Mock<ITrade>.With(fun x -> <@ x.Aggressor --> null @>)

// Tuples

type IMath =
    abstract DivRem : float * float -> (float * float)

let ``can setup member to return tuple as value`` () =
    Mock<IMath>().Setup(fun x -> <@ x.DivRem(any(),any()) @>).Returns((1.0,2.0))

let ``cannot setup member to return tuple as null`` () =
    // Error: No overload match for method 'Returns'
    Mock<IMath>().Setup(fun x -> <@ x.DivRem(any(),any()) @>).Returns(null)

let ``cannot mock with member returning tuple as null`` () =
    // Error: The type '(float*float)' does not have 'null' as a proper value
    Mock<IMath>.With(fun x -> <@ x.DivRem(any(),any()) --> null @>)