Is yield return in C# thread-safe?

I have the following piece of code:

private Dictionary<object, object> items = new Dictionary<object, object>;
public IEnumerable<object> Keys
{
    get
    {
        foreach (object key in items.Keys)
        {
            yield return key;
        }
    }
}

Is this thread-safe? If not do I have to put a lock around the loop or the yield return?

Here is what I mean:

Thread1 accesses the Keys property while Thread2 adds an item to the underlying dictionary. Is Thread1 affected by the add of Thread2?