BooleanDifference Crash

As far as I can tell, there is nothing wrong with this simple file - no bad objects. if I open it in rhino 6 WIP (Version 6 WIP (6.15.19113.12066, 2019-04-23)), and then type “BooleanDifference” and press enter (with nothing selected), it will either crash immediately, or sometimes wait until i have selected target (the purple object) and operator objects (the small blue tabs) and pressed enter.

the same operation on the same objects in Rhino 5.5.3 for mac works fine.

4max TA duct.3dm (531.2 KB)