After running rndc stop shortly after starting the server,

2018-02-21.12:58:47.215 general: critical: rpz.c:2202: REQUIRE(rpz != ((void *)0)) failed
2018-02-21.12:58:47.215 general: critical: exiting (due to assertion failure)

#0  0x00007fc47dcc2067 in __GI_raise (sig=sig@entry=6)
    at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
#1  0x00007fc47dcc3448 in __GI_abort () at abort.c:89
#2  0x000055ad8a459439 in assertion_failed (file=0x55ad8a6e0e8f "rpz.c", line=2202,
    type=2325208112, cond=0x55ad8a6e0efa "rpz != ((void *)0)") at ./main.c:248
#3  0x000055ad8a64f9ba in isc_assertion_failed (
    file=file@entry=0x55ad8a6e0e8f "rpz.c", line=line@entry=2202,
    type=type@entry=isc_assertiontype_require,
    cond=cond@entry=0x55ad8a6e0efa "rpz != ((void *)0)") at assertions.c:49
#4  0x000055ad8a5bc932 in dns_rpz_add (rpzs=0x7fc470200500, rpz_num=2 '\002',
    src_name=<optimized out>) at rpz.c:2202
#5  0x000055ad8a5bd08b in update_quantum (task=0x5045, event=0x0) at rpz.c:1914
#6  0x000055ad8a676177 in dispatch (manager=0x7fc47f89f010) at task.c:1138
#7  run (uap=0x7fc47f89f010) at task.c:1310
#8  0x00007fc47e3a7064 in start_thread (arg=0x7fc47a72c700) at pthread_create.c:309
#9  0x00007fc47dd7562d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

I have not investigated this in any detail yet - I'll add more info to this issue if/when I have it.