Saying the contract static checker that a extension method not allow null?

Question

 Contract.Requires(completeURL.IsUri()); // Error: Contract.Requires(completeURL != null)

Can I do this without Contract.Assume()?
I mean something like this:

    [Pure]
    [DefinitelyNotANullStringAfterThatMethod]
    public static bool IsUri(this string str)
    {
        return Uri.IsWellFormedUriString(str, UriKind.Absolute);
    }

Answer 1

    [Pure]
    public static bool IsUri(this string str)
    {
        Contract.Ensures(!Contract.Result<bool>() || str != null); // by Dan Bryant
        return Uri.IsWellFormedUriString(str, UriKind.Absolute);
    }

Maybe?