null array-valued attribute causes the compiler internal error

[bleis-tift]

Description:

Repro Steps:

open System

type Attr([<ParamArray>] x: obj[]) =
  inherit Attribute()

[<Attr(null)>]
let f () = ()

Expected:
Compilation successful.

Actual:
Compilation error.

error FS0193: internal error: not a nominal type

Version:
I tried VS 2013 only.

Link:
compilation result on ideone

Workaround:
use type annotation.

open System

type Attr([<ParamArray>] x: obj[]) =
  inherit Attribute()

[<Attr(null: string)>]
let f () = ()