vtbd0: hard error

Lately whenever I try to buildworld on my main development VM, I get
"vtbd0: hard error" errors, and inevitably UFS panics soon afterwards.
Does anybody else see this?  vtbd0 is backed by a zvol that apparently has
no errors, and there's nothing in the host's dmesg.  What else could be
cause these errors on a virtio device?

vtbd0: hard error cmd=write 56117912-56117919
g_vfs_done():vtbd0p2[WRITE(offset=28731822080, length=4096)]error = 5
vtbd0: hard error cmd=write 56117880-56117911
g_vfs_done():vtbd0p2[WRITE(offset=28731805696, length=16384)]error = 5
vtbd0: hard error cmd=write 78822-78835
g_vfs_done():vtbd0p2[WRITE(offset=39808000, length=7168)]error = 5
panic: brelse: inappropriate B_PAGING or B_CLUSTER bp 0xfffffe00109eb880
cpuid = 0
time = 1582524584
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame
0xfffffe004dd64850
vpanic() at vpanic+0x185/frame 0xfffffe004dd648b0
panic() at panic+0x43/frame 0xfffffe004dd64910
brelse() at brelse+0x653/frame 0xfffffe004dd64970
bufdone() at bufdone+0x4a3/frame 0xfffffe004dd649f0
g_io_deliver() at g_io_deliver+0x21d/frame 0xfffffe004dd64a40
g_io_deliver() at g_io_deliver+0x21d/frame 0xfffffe004dd64a90
g_disk_done() at g_disk_done+0xed/frame 0xfffffe004dd64ad0
vtblk_vq_intr() at vtblk_vq_intr+0xce/frame 0xfffffe004dd64b20
ithread_loop() at ithread_loop+0x2ae/frame 0xfffffe004dd64bb0
fork_exit() at fork_exit+0x80/frame 0xfffffe004dd64bf0
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe004dd64bf0
--- trap 0, rip = 0, rsp = 0, rbp = 0 ---
KDB: enter: panic