Bridges don't work: Non-fatal assertion !(exit_ei == NULL) failed in onion_pick_cpath_exit

When running bridges-min and bridges+ipv6-min in chutney on macOS 10.13, Apple clang 9.0.0 with Xcode 9.2:

Apr 03 08:16:13.578 [info] circuit_predict_and_launch_new: Have 0 clean circs (0 internal), need another exit circ.
Apr 03 08:16:13.578 [info] choose_good_exit_server_general: Found 1 servers that might support 0/0 pending connections.
Apr 03 08:16:13.579 [info] choose_good_exit_server_general: Chose exit server '$2E832C796F35B96C3854941B01FF4E51E0968E24~test002r at 127.0.0.1'
Apr 03 08:16:13.580 [warn] tor_bug_occurred_: Bug: src/or/circuitbuild.c:2390: onion_pick_cpath_exit: Non-fatal assertion !(exit_ei == NULL) failed. (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.585 [warn] Bug: Non-fatal assertion !(exit_ei == NULL) failed in onion_pick_cpath_exit at src/or/circuitbuild.c:2390. Stack trace: (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.586 [warn] Bug:     0   tor                                 0x00665f83 log_backtrace + 67 (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.587 [warn] Bug:     1   tor                                 0x006e0ad7 tor_bug_occurred_ + 503 (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.587 [warn] Bug:     2   tor                                 0x0011843d circuit_establish_circuit + 18621 (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.588 [warn] Bug:     3   tor                                 0x00186560 circuit_launch_by_extend_info + 992 (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.589 [warn] Bug:     4   tor                                 0x0017f868 circuit_build_needed_circs + 3784 (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.590 [warn] Bug:     5   tor                                 0x0044b33c second_elapsed_callback + 1468 (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.591 [warn] Bug:     6   tor                                 0x0075b2f1 periodic_timer_cb + 129 (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.592 [warn] Bug:     7   libevent-2.1.6.dylib                0x0104f6f3 event_process_active_single_queue + 1289 (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.593 [warn] Bug:     8   libevent-2.1.6.dylib                0x0104bf12 event_base_loop + 1109 (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.594 [warn] Bug:     9   tor                                 0x00449fd9 do_main_loop + 3289 (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.595 [warn] Bug:     10  tor                                 0x00453990 tor_run_main + 704 (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.596 [warn] Bug:     11  tor                                 0x006585a7 tor_main + 71 (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.597 [warn] Bug:     12  tor                                 0x000a3546 main + 38 (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.598 [warn] Bug:     13  libdyld.dylib                       0xa75576e1 start + 1 (on Tor 0.3.4.0-alpha-dev 3df954549232bf55)
Apr 03 08:16:13.599 [info] circuit_mark_for_close_: Circuit 0 (id: 2) marked for close at src/or/circuitbuild.c:479 (orig reason: -2, new reason: 0)
Apr 03 08:16:13.600 [info] circuit_free_: Circuit 0 (id: 2) has been freed.

Other networks also log this bug but don't fail.

$ clang --version
Apple LLVM version 9.0.0 (clang-900.0.39.2)
Target: x86_64-apple-darwin17.3.0
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