IGrouping<TKey, TElement> implementation and null keys

Question

I'm converting code in preparation to C#9, in particular the below class implementing IGrouping<TKey, TElement>:

public class Grouping<TKey, TElement> : IGrouping<TKey, TElement>
{
    public TKey Key { get; }

    public Grouping(TKey key, IEnumerable<TElement> elements)
    {
        Key = key;
        _elements = elements;
    }
    private readonly IEnumerable<TElement> _elements;

    ...
}

Lookups support null keys so in theory the constructor should be: public Grouping(TKey? key, IEnumerable<TElement> elements)

but then Key = key; raises a warning since Key is not defined as TKey? (as per the interface's specs).

I feel like something missing in the interface's definition but maybe am I missing something?