I am trying to define an enclosure as the intersection of a rectangular cuboid and two cylinders, and I would like to add fillets to all edges, except four.
However, freecad does not allow me to: even if the edges/faces are selected, and I choose "Fillet" from the menu, I get the "Select an edge, face or body from an active body." message, but I can't activate the body, because as soon as I try to select an edge, the top-level object, a common, will become active.

I have also tried making the cylinder as a surface of revolution, or taking it from the standard objects in the Part workbench, but that does not change the situation.
In any case, I would appreciate, if someone could shed some light on the problem. I am quite sure I miss something totally trivial.
Thanks,
Zoltán
OS: Ubuntu 18.04 LTS
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.17.13522 (Git)
Build type: None
Branch: releases/FreeCAD-0-17
Hash: 3bb5ff4e70c0c526f2d9dd69b1004155b2f527f2
Python version: 2.7.15rc1
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 7.2.0
Locale: English/UnitedStates (en_US)