Known bug affecting versions: 8.5.1, 8.5.2, 8.5.3, 8.6.0.
Fixed in 8.6.1.Update5; 8.6.0.Update13; 8.5.3.Update18.
Occurs when selecting two or more joists using Joist Advanced Material Selection.
Change material and click OK. Error displays as shown.
Work-around is to use Joist Material selection instead.