EngineControl::device_changed assert(backend)

So I just did a git pull to ardour-3.5.4553, distcleaned and rebuilt. 
Before I go deep into the bowels of full step-through gdb session, I 
thought I'd ask if this was maybe just a one-off.

Running ./ardev core dumped so I ran ./ardbg

This occurs during the Session Open dialog. Jack version: 1.9.9.5 
running @44100

#0  0x0000003848a35877 in raise () from /lib64/libc.so.6
#1  0x0000003848a36f68 in abort () from /lib64/libc.so.6
#2  0x0000003848a2e7d6 in __assert_fail_base () from /lib64/libc.so.6
#3  0x0000003848a2e882 in __assert_fail () from /lib64/libc.so.6
#4  0x00000000012511e9 in EngineControl::device_changed (this=0x22dac30) 
at ../gtk2_ardour/engine_dialog.cc:906
#5  0x0000000001257512 in EngineControl::set_desired_sample_rate 
(this=0x22dac30, sr=0) at ../gtk2_ardour/engine_dialog.cc:1865
#6  0x0000000000f84d2e in ARDOUR_UI::do_audio_midi_setup 
(this=0x2034030, desired_sample_rate=0) at ../gtk2_ardour/ardour_ui.cc:4520
#7  0x0000000000fb324f in boost::_mfi::mf1<int, ARDOUR_UI, unsigned 
int>::operator() (this=0x242a728, p=0x2034030, a1=0) at 
/usr/include/boost/bind/mem_fn_template.hpp:165
#8  0x0000000000faaec6 in 
boost::_bi::list2<boost::_bi::value<ARDOUR_UI*>, boost::arg<1> 
 >::operator()<int, boost::_mfi::mf1<int, ARDOUR_UI, unsigned int>, 
boost::_bi::list1<unsigned int&> > (this=0x242a738, f=..., a=...) at 
/usr/include/boost/bind/bind.hpp:303
#9  0x0000000000fa51f2 in boost::_bi::bind_t<int, boost::_mfi::mf1<int, 
ARDOUR_UI, unsigned int>, 
boost::_bi::list2<boost::_bi::value<ARDOUR_UI*>, boost::arg<1> > 
 >::operator()<unsigned int> (this=0x242a728, a1=@0x7fffffffb694: 0) at 
/usr/include/boost/bind/bind_template.hpp:32
#10 0x0000000000f9efab in 
boost::detail::function::function_obj_invoker1<boost::_bi::bind_t<int, 
boost::_mfi::mf1<int, ARDOUR_UI, unsigned int>, 
boost::_bi::list2<boost::_bi::value<ARDOUR_UI*>, boost::arg<1> > >, int, 
unsigned int>::invoke (function_obj_ptr=..., a0=0) at 
/usr/include/boost/function/function_template.hpp:132
#11 0x00007ffff7178c42 in boost::function1<int, unsigned 
int>::operator() (this=0x242a720, a0=0) at 
/usr/include/boost/function/function_template.hpp:767
#12 0x00007ffff71777fb in PBD::Signal1<int, unsigned int, 
PBD::OptionalLastValue<int> >::operator() (this=0x1f19ee0 
<ARDOUR::Session::AudioEngineSetupRequired>, a1=0)
     at /home/david/checkouts/a3/build/libs/pbd/pbd/signals_generated.h:483
#13 0x00007ffff747f1ee in ARDOUR::Session::ensure_engine 
(this=0x2421c20, desired_sample_rate=0) at ../libs/ardour/session.cc:378
#14 0x00007ffff747d054 in ARDOUR::Session::Session (this=0x2421c20, 
eng=..., fullpath="/home/david/a3-projects/test44k", 
snapshot_name="test44k", bus_profile=0x7fffffffbc30,
     mix_template="") at ../libs/ardour/session.cc:279
#15 0x0000000000f7cc31 in ARDOUR_UI::build_session (this=0x2034030, 
path="/home/david/a3-projects/test44k", snap_name="test44k", 
bus_profile=...) at ../gtk2_ardour/ardour_ui.cc:3062
#16 0x0000000000f7a87a in ARDOUR_UI::build_session_from_dialog 
(this=0x2034030, sd=..., session_path="/home/david/a3-projects/test44k", 
session_name="test44k")
     at ../gtk2_ardour/ardour_ui.cc:2635
#17 0x0000000000f7b895 in ARDOUR_UI::get_session_parameters 
(this=0x2034030, quit_on_cancel=false, should_be_new=false, 
load_template="") at ../gtk2_ardour/ardour_ui.cc:2862
#18 0x0000000000f71b8b in ARDOUR_UI::starting (this=0x2034030) at 
../gtk2_ardour/ardour_ui.cc:866
#19 0x00007ffff5deb4c0 in Gtkmm2ext::UI::run (this=0x2034030, 
old_receiver=...) at ../libs/gtkmm2ext/gtk_ui.cc:275
#20 0x000000000135f693 in main (argc=1, argv=0x7fffffffd6c8) at 
../gtk2_ardour/main.cc:303