Problem with Viewers seeing the calendar

Does anyone else have a problem with assigning Viewers and then them not being able to see the calendar properly? 

I'm an admin but I added one person as a Viewer however when they open the calendar, it shows the bookings briefly but then they disappear as if it's realised they don't have permission to view, even though they should do. I don't really want to have to add them as an admin but it seems that will be the only way I can get them to view the calendar?