Panic in getblkx() booting from disc1.iso in Qemu VM

I know booting from the cd/dvd iso worked fairly recently, but today booting a 
new build of -CURRENT in a Qemu VM resulted in a panic:

FreeBSD 13.0-CURRENT 19a6ceb89db(HEAD) GENERIC amd64
FreeBSD clang version 7.0.1 (tags/RELEASE_701/final 349250) (based on LLVM 
7.0.1)
WARNING: WITNESS option enabled, expect reduced performance.
VT(efifb): resolution 800x600
CPU: QEMU Virtual CPU version 2.5+ (2994.86-MHz K8-class CPU)
  Origin="AuthenticAMD"  Id=0x663  Family=0x6  Model=0x6  Stepping=3
  Features=0x783fbfd<FPU,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,MMX,FXSR,SSE,SSE2>
  Features2=0x80002001<SSE3,CX16,HV>
  AMD Features=0x20100800<SYSCALL,NX,LM>
  AMD Features2=0x5<LAHF,SVM>
  SVM: NAsids=16
Hypervisor: Origin = "TCGTCGTCGTCG"
real memory  = 8589934592 (8192 MB)
avail memory = 8227643392 (7846 MB)

...

WARNING: WITNESS option enabled, expect reduced performance.
Trying to mount root from cd9660:/dev/iso9660/13_0_CURRENT_AMD64_DVD [ro]...
panic: bsize == 0, check bo->bo_bsize
cpuid = 0
time = 2
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe0041189270
vpanic() at vpanic+0x1b4/frame 0xfffffe00411892d0
panic() at panic+0x43/frame 0xfffffe0041189330
getblkx() at getblkx+0x807/frame 0xfffffe00411893f0
breadn_flags() at breadn_flags+0x3d/frame 0xfffffe0041189460
cd9660_blkatoff() at cd9660_blkatoff+0x53/frame 0xfffffe00411894d0
cd9660_vget_internal() at cd9660_vget_internal+0x26b/frame 0xfffffe0041189550
vfs_domount() at vfs_domount+0x7d3/frame 0xfffffe0041189770
vfs_donmount() at vfs_donmount+0x7b9/frame 0xfffffe0041189810
kernel_mount() at kernel_mount+0x58/frame 0xfffffe0041189860
parse_mount() at parse_mount+0x469/frame 0xfffffe00411899a0
vfs_mountroot() at vfs_mountroot+0x67f/frame 0xfffffe0041189b10
start_init() at start_init+0x28/frame 0xfffffe0041189bb0
fork_exit() at fork_exit+0x84/frame 0xfffffe0041189bf0
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe0041189bf0
--- trap 0, rip = 0, rsp = 0, rbp = 0 ---
KDB: enter: panic
[ thread pid 1 tid 100002 ]
Stopped at      kdb_enter+0x3b: movq    $0,kdb_why
db> 


-- 
Rebecca