Cryptic F＃error - 签名和实现不兼容，因为type参数的声明需要约束为“T = null”

Question

我有一小段F＃代码产生了一个我无法弄清楚的神秘错误......

namespace epic.View

open System
open FSharp.Desktop.UI

[<AbstractClass>]
type public FieldModelBase<'ValueType>() = 
    inherit Model() 
    let mutable fieldCaption: string = null
    let mutable fieldValue: 'ValueType = null
    [<DerivedProperty>]
    member this.Caption with public get() = sprintf "%s" fieldCaption and set value = fieldCaption <- sprintf "%s" value; this.NotifyPropertyChanged "Caption"
    [<DerivedProperty>]
    member inline this.Value with public get() = fieldValue and set value = fieldValue <- value; this.NotifyPropertyChanged "Value"

会产生以下错误：

  

签名和实现不兼容，因为   声明类型参数＆＃39; ValueType＆＃39;需要约束   表单＆＃39; ValueType：null

这是什么意思，我该如何解决？看起来我应该在＆＃39; ValueType类型参数上添加一个类型约束，要求它是......可变的？是吗？