Something I’ve done before but just won’t do it now!
Here’s an example. Creating a cylinder and shelling that is as expected, but loft the 2 circles (different diameters) and clicking on the top face to shell says ‘invalid selection’. After fiddling about for ages, just can’t get it to work - anyone see what I’m doing wrong! 
Thanks. [Version 5.760.0 on Mac]
You might be selecting the upper circle instead of the cylinder.
Hide the sketch first.
You are correct! Yes, hiding the sketch sorts it out. Funny, that I haven’t run into this problem before when shelling out. Thanks again
Probably because Shapr hides a sketch automatically after using up all the profiles in it.
Work around when I had this problem was to extrude up a little like 0.01 then shell should work hope this helps
Thanks, good idea. It’s just annoying that the sketch is picked up in the first place rather than the body!
Also, works with shelling odd shapes