Why extension method cannot exist in a nested class?

Question

I have a static class having a static method which is an extension method, like this:

public static class N
{
    public static int Plus(this int i)
    {
        return i + 10;
    }
}

When I try to move this class into a class, to be a nested class, compiler reports error.

Why C# has this kind of limitation for extension method, what for? I don't see any danger that a nested class could contain extension method.

Thanks!