I'm getting an error, 

Challenge Not yet complete... here's what's wrong: 
The 'IsShiftVolunteer' field formula on the 'Volunteer Shift Workers' is not resulting in a check box when the logged in user is the volunteer, and unchecked when they are not.

I did check with two users and two volunteer shift worker records, and it works as expected. 

I entered the forumla as 

!ISBLANK(Volunteer__r.Id) && $User.Id = Volunteer__r.Id

I'm not sure what I'm missing here.