floating point exception creating a very large array

Running these commands:

t=$(mktemp -d)
mount -t tmpfs tmpfs $t
truncate -s 2400T $t/0.img
truncate -s 2400T $t/1.img
l1=$(losetup -f --show $t/0.img)
l2=$(losetup -f --show $t/1.img)
devname=/dev/md/test-$(uuidgen)
mdadm --verbose --create --metadata default --level raid1 --run -n 2 \
      --assume-clean $devname $l1 $l2

results in:

mdadm: size set to 2576980245504K
mdadm: automatically enabling write-intent bitmap on large array
Floating point exception

A little light gdb-ing shows that the crash is in super1.c:calc_bitmap_size where __le32_to_cpu(bms->chunksize) is 0. The backtrace is this:

#0 0x00005555555a2126 in bitmap_bits (array_size=5153960491008, chunksize=0) at mdadm.h:1419
#1 0x00005555555a21a6 in calc_bitmap_size (bms=0x55555562d000, boundary=4096) at super1.c:179
#2 0x00005555555a4b05 in getinfo_super1 (st=0x555555620360, info=0x555555625d70, map=0x0) at super1.c:1043
#3 0x000055555557f18d in Create (st=0x555555620360, mddev=0x7fffffffd940 "/dev/md/test-2acca3e1-7896-4a4c-a9cb-c603bb429e3c",
    name=0x7fffffffd948 "test-2acca3e1-7896-4a4c-a9cb-c603bb429e3c", uuid=0x0, subdevs=2, devlist=0x555555620450,
    s=0x7fffffffdef0, c=0x7fffffffdf40, data_offset=1) at Create.c:913
#4 0x000055555555f900 in main (argc=14, argv=0x7fffffffe468) at mdadm.c:1586

super1.c:1043 is this line:

   unsigned long long size = calc_bitmap_size(bsb, 4096);

bsb doesn't seem totally wrong, the magic is right here:

(gdb) p/x bsb[0]
$5 = {magic = 0x6d746962, version = 0x4, uuid = {0x5, 0x11, 0x6c, 0xd9, 0x8, 0x9e, 0x3c, 0xd3, 0x74, 0xd4, 0x68, 0xae, 0x43, 0x90,
    0x2b, 0x45}, events = 0x0, events_cleared = 0x0, sync_size = 0x4affffbf800, state = 0x0, chunksize = 0x0, daemon_sleep = 0x5,
  write_behind = 0x0, sectors_reserved = 0x0, nodes = 0x0, cluster_name = {0x0 <repeats 64 times>}, pad = {
    0x0 <repeats 120 times>}}

But clearly chunksize being 0 is bogus.