如何模拟返回任务(IList(PasswordChangeHistory))的异步方法

这就是我想要模仿的方法:

Function ReturnPasswordHistoryAsync(passwordChangeHistory As PasswordChangeHistory) As Task(Of IList(Of PasswordChangeHistory))

这是我的单元测试:

<TestMethod()> Public Async Function ValidateNewPassword_NewPasswordHasAlreadyBeenUsed_PasswordIsNotValid() As Task
        'Arrange
        Dim newPass = "newPass"
        Dim oldPass = "oldPass"
        Dim confirmPass = newPass
        Dim passwordValid As PasswordValid
        Dim ret As Task(Of IList(Of PasswordChangeHistory))

        'Errors here with null object reference error
        _mockChangePasswordRepo.Setup(Function(x) x.ReturnPasswordHistoryAsync(New PasswordChangeHistory())).Returns(ret)

        'Act
        passwordValid = Await _changePasswordManager.ValidateNewPassword(oldPass, newPass, confirmPass, Nothing)

        'Assert
        Assert.IsFalse(passwordValid.IsValid, "New password and old passord equal one another yet the test passed")
    End Function

Task(Of IList(Of PasswordChangeHistory))