Operator 'is' with type, retrieved by Reflection

How to implement operator 'is' with a type, that I got from reflection (since that type marked as internal)? My code:

    Type someType = someObj.GetType();
    Type runtimeType = Assembly.GetAssembly(typeof(string)).GetType("System.RuntimeType");  //Marked as internal

    if (someType is runtimeType)    //Not compiling...
    {
        //To do something
    }

I wrote some hackery code, but may be there is a better solution than that:

    private static bool _isThat<T, K>(T obj) { return obj is K; }

    private static readonly MethodInfo _isThatMethodInfo = typeof(Program).GetMethod("_isThat", 
        BindingFlags.NonPublic | BindingFlags.Static);

    public static bool IsThatType<T>(T obj, Type type)
    {
        return type == null || obj == null ? false
            : (bool)_isThatMethodInfo.MakeGenericMethod(typeof(T), type).Invoke(null, new object[] { obj });
    }

Unfortunately, method IsSubclassOf returns false if someType == runtimeType.