Code Contracts [Type]implements interface method {Interface.Method} thus cannot add requires

I have the following scenario:

public interface ISomething
{
    void DoStuff();
    //...
}

public class Something : ISomething
{
    private readonly ISomethingElse _somethingElse;
    //...

    public Something (ISomethingElse somethingElse)
    {
         Contract.Requires(somethingElse != null);
        _somethingElse = somethingElse;
    }

    public void DoStuff()
    {
        // *1* Please look at explanation / question below
        _somethingElse.DoThings();
    }
 }

At line 1 and with the static checker on, I'll get a warning saying that _somethingElse is possibly null, and if I add a contract it will give me the error

[Type]implements interface method {Interface.Method} thus cannot add requires

What's the best thing to do here? Options I see include

1. a guard clause, though it seems a bit extreme
2. a Contract.Assume
3. a hidden third option that I haven't thought of

Please note the field is readonly so after setting the value in the constructor it is not possible to change. Thus, the warning from code contracts seems a bit irrelevant.