What does it mean when zdb -R x:xxx:xxx:g crashes?

[Zaphod Beeblebrox]

So... another barrier in figuring out my zfs problem is that zdb crashes
when asked to print the gang block header:

[1:97:397]root at virtual:/vr1/tmp/diag> zdb -AAA -R vr2 0:94048dc9000:24000:g
Found vdev type: raidz
Assertion failed: (zio->io_error == 0 || (zio->io_flags &
ZIO_FLAG_CANFAIL)), file
/usr/src/cddl/lib/libzpool/../../../sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zio.c,
line 3297.
Abort trap (core dumped)

Now... this trace looks odd to me: there are no bookmarks in the
filesystem... neither are there snapshots for this specific filesystem.

(gdb) bt
#0  0x0000000801cb26ca in thr_kill () from /lib/libc.so.7
#1  0x0000000801d87149 in abort () from /lib/libc.so.7
#2  0x0000000801920e21 in zio_init () from /lib/libzpool.so.2
#3  0x0000000801927e0e in zbookmark_is_before () from /lib/libzpool.so.2
#4  0x0000000801922df7 in zio_execute () from /lib/libzpool.so.2
#5  0x0000000801927f11 in zbookmark_is_before () from /lib/libzpool.so.2
#6  0x0000000801922df7 in zio_execute () from /lib/libzpool.so.2
#7  0x0000000801927f11 in zbookmark_is_before () from /lib/libzpool.so.2
#8  0x0000000801922df7 in zio_execute () from /lib/libzpool.so.2
#9  0x0000000801927f11 in zbookmark_is_before () from /lib/libzpool.so.2
#10 0x0000000801922df7 in zio_execute () from /lib/libzpool.so.2
#11 0x0000000801927f11 in zbookmark_is_before () from /lib/libzpool.so.2
#12 0x0000000801922df7 in zio_execute () from /lib/libzpool.so.2
#13 0x000000080191b8d9 in taskq_create () from /lib/libzpool.so.2
#14 0x0000000800e814f5 in pthread_create () from /lib/libthr.so.3
#15 0x00007ffff6fb9000 in ?? ()
Cannot access memory at address 0x7ffff71b9000

Help?