Active pattern broken in F# 3.0

This active pattern compiles with F# 2.0:

let (|Value|_|) value = // 'a -> 'T option
  match box value with
  | :? 'T as x -> Some x
  | _ -> None

but, in F# 3.0, emits the error:

Active pattern '|Value|_|' has a result type containing type variables that are not determined by the input. The common cause is a [sic] when a result case is not mentioned, e.g. 'let (|A|B|) (x:int) = A x'. This can be fixed with a type constraint, e.g. 'let (|A|B|) (x:int) : Choice = A x'

I tried:

let (|Value|_|) value : 'T option = ...

and:

let (|Value|_|) (value: 'U) = ...

How can it be fixed?

Environments: Visual Studio 2012 (RTM) and FSI v11.0.50727.1

EDIT: Here's a simpler repro:

let (|X|) x = unbox x