Spectest sometimes fail with small nursery

gistfile1.txt

ogbert@dogbert-VirtualBox ~/repos/rakudo $ gdb --args /home/dogbert/repos/rakudo/install/bin/moar --libpath="/home/dogbert/repos/rakudo/install/share/nqp/lib" --libpath="." /home/dogbert/repos/rakudo/perl6.moarvm t/spec/S15-nfg/many-threads.t
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Reading symbols from /home/dogbert/repos/rakudo/install/bin/moar...done.
(gdb) break MVM_panic
Function "MVM_panic" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (MVM_panic) pending.
(gdb) r
Starting program: /home/dogbert/repos/rakudo/install/bin/moar --libpath=/home/dogbert/repos/rakudo/install/share/nqp/lib --libpath=. /home/dogbert/repos/rakudo/perl6.moarvm t/spec/S15-nfg/many-threads.t
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1".
1..4
[New Thread 0xb65dfb40 (LWP 8817)]
[New Thread 0xb5bffb40 (LWP 8818)]
[New Thread 0xb51ffb40 (LWP 8819)]
[New Thread 0xb49feb40 (LWP 8820)]
[Switching to Thread 0xb49feb40 (LWP 8820)]

Breakpoint 1, MVM_panic (exitCode=1, messageFormat=0xb726850c "Collectable %p in fromspace accessed") at src/core/exceptions.c:781
781	    fprintf(stderr, "MoarVM panic: ");
(gdb) bt
#0  MVM_panic (exitCode=1, messageFormat=0xb726850c "Collectable %p in fromspace accessed") at src/core/exceptions.c:781
#1  0xb72671a3 in finish_store () from ./dynext/libperl6_ops_moar.so
#2  0xb7cc922a in MVM_interp_run (tc=0x98b0928, initial_invoke=0xb7cf34ac <thread_initial_invoke>, invoke_data=0x98aef10) at src/core/interp.c:2871
#3  0xb7cf35ee in start_thread (data=0x98aef10) at src/core/threads.c:77
#4  0xb7de4f97 in uv.thread_start () from //home/dogbert/repos/rakudo/install/lib/libmoar.so
#5  0xb7a00f72 in start_thread (arg=0xb49feb40) at pthread_create.c:312
#6  0xb7b48f8e in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:129
(gdb) frame 2
#2  0xb7cc922a in MVM_interp_run (tc=0x98b0928, initial_invoke=0xb7cf34ac <thread_initial_invoke>, invoke_data=0x98aef10) at src/core/interp.c:2871
2871	                    spec->store_unchecked(tc, cont, obj);
(gdb) p MVM_dump_backtrace(tc)
   at <unknown>:1  (./CORE.setting.moarvm:push-until-lazy)
 from SETTING::src/core/List.pm:243  (./CORE.setting.moarvm:reify-until-lazy)
 from SETTING::src/core/List.pm:691  (./CORE.setting.moarvm:)
 from SETTING::src/core/List.pm:689  (./CORE.setting.moarvm:push-until-lazy)
 from SETTING::src/core/List.pm:256  (./CORE.setting.moarvm:reify-until-lazy)
 from SETTING::src/core/List.pm:536  (./CORE.setting.moarvm:elems)
 from SETTING::src/core/Uni.pm:9  (./CORE.setting.moarvm:new)
 from t/spec/S15-nfg/many-threads.t:7  (<ephemeral file>:)
 from SETTING::src/core/List.pm:1377  (./CORE.setting.moarvm:infix:<xx>)
 from t/spec/S15-nfg/many-threads.t:7  (<ephemeral file>:)
 from SETTING::src/core/Promise.pm:175  (./CORE.setting.moarvm:)
 from SETTING::src/core/ThreadPoolScheduler.pm:163  (./CORE.setting.moarvm:)
 from SETTING::src/core/ThreadPoolScheduler.pm:51  (./CORE.setting.moarvm:)
 from SETTING::src/core/ThreadPoolScheduler.pm:44  (./CORE.setting.moarvm:)
 from SETTING::src/core/ThreadPoolScheduler.pm:41  (./CORE.setting.moarvm:)
 from SETTING::src/core/Thread.pm:30  (./CORE.setting.moarvm:THREAD-ENTRY)
$1 = void