Boolean difference resulting in fatal error

[Crawdad]

I'm trying to subtract six cylinders from a shape that was create by doing mirror on a building block, and then mirror again in a perpendicular direction. Four of my cylinders can 'difference' just fine, but the two that straddle the plane of the previous mirror operation give me a crash dump like this:

Dump written 2019-9-8_21-48
Version: 2.2.4
OS: Mac OS X (Darwin 18.7.0 x86_64)
GPU: NVIDIA Corporation | NVIDIA GeForce GT 755M OpenGL Engine
Window: geom
Reason: no_loop

Stack trace:
[{wings_bool,make_directed,6,[{file,"wings_bool.erl"},{line,1174}]},
{wings_bool,make_directed,3,[{file,"wings_bool.erl"},{line,1141}]},
{wings_bool,build_vtx_loops,1,[{file,"wings_bool.erl"},{line,1111}]},
{wings_bool,merge_1,5,[{file,"wings_bool.erl"},{line,196}]},
{wings_bool,find_intersect_1,3,[{file,"wings_bool.erl"},{line,169}]},
{wings_bool,find_intersect,3,[{file,"wings_bool.erl"},{line,159}]},
{wings_sel,dfold_1,5,[{file,"wings_sel.erl"},{line,236}]},
{wings_bool,sub,2,[{file,"wings_bool.erl"},{line,73}]},
{wings_bool,'-sub/1-fun-0-',2,[{file,"wings_bool.erl"},{line,60}]},
{wings,handle_event_3,2,[{file,"wings.erl"},{line,414}]},
{wings_wm,handle_event,3,[{file,"wings_wm.erl"},{line,984}]},
{wings_wm,send_event,2,[{file,"wings_wm.erl"},{line,951}]},
{wings_wm,do_dispatch,2,[{file,"wings_wm.erl"},{line,852}]},
{wings_wm,dispatch_event,1,[{file,"wings_wm.erl"},{line,761}]},
{lists,all,2,[{file,"lists.erl"},{line,1213}]},
{wings_wm,redraw_win,1,[{file,"wings_wm.erl"},{line,889}]},
{lists,foreach,2,[{file,"lists.erl"},{line,1338}]},
{wings_wm,redraw_all,0,[{file,"wings_wm.erl"},{line,868}]},
{wings,init_part2,2,[{file,"wings.erl"},{line,106}]},
{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,249}]}]

=========

There are two coincident internal faces that intersect the cylinder I'm trying to subtract, left I suppose by the mirror function. I can't bridge them to reduce them to one: "Faces must not be neighbors" is the error. Neither of these faces needs to exist, since they are both completely internal.