I forgot a `do`. This compiles (and works):

```F#
type AutoFoqCustomization(relay: ISpecimenBuilder) =
    do if relay = null then raise (ArgumentNullException("relay"))

    new() = 
        AutoFoqCustomization(
            FilteringSpecimenBuilder(
                MethodInvoker(
                    FoqMethodQuery()),
                AbstractTypeSpecification()))

    member this.Relay = relay
```