Why is this code not threadsafe?

I have a piece of code like this:

public class UserCache
{
    private Dictionary<int, User> _users = new Dictionary<int, User>();

    public User GetUser(int id)
    {
        User u = null;

        lock (_users)
        {
            if (_users.containsKey(id))
                return _users[id];
        }

        //The below line is threadsafe, so no worries on that.
        u = RetrieveUser(id); // Method to retrieve from database;

        lock (_users)
        {
            _users.Add(id, u);
        }

        return u;
    }
}

I'm locking the access to dictionary, however someone in my team told that its still not threadsafe(without an explanation). Question is - do you think this is thread safe at all?

Edit: Forgot to ask,what a solution would look like.Please note that I'mnot looking to lock the whole method, as the retrieve user is a time consuming operation.