Can't refer to `UserId` in an `Id` field

This QQ breaks with an error that `UserId` is not in scope.

```haskell
share [ mkPersist sqlSettings ] [persistLowerCase|

User
    name    String

Profile
    Id      UserId
    email   String
|]
```

However, using `UserId` is *usually* fine in other contexts. So why isn't this working?