How to pass null/None to Procedure?

``` sql
CREATE PROCEDURE dbo.test (
    @OptionalParam INT = NULL
)
AS 

SELECT 1
```

``` fsharp
//Snip...
[ for r in db.Procedures.test.Invoke(None).ResultSet
                  -> r.MapTo<TestResult> () ]
//Error: Expression was expected to have type int but here has type Option<int>
```
