VFS panic

[Robert Swindells <rjs%fdy2.co.uk@localhost>]

I'm getting a panic at startup on an evbearmv7hf-el system:

...
Starting sshd.
Starting inetd.
Starting cron.
Wed Feb 20 21:23:46 GMT 2019
panic: kernel diagnostic assertion "mp != dead_rootmount" failed: file "../../../../kern/vfs_trans.c", line 680 
cpu1: Begin traceback...
0x9ac8dd54: netbsd:db_panic+0x14
0x9ac8dd6c: netbsd:vpanic+0x194
0x9ac8dd84: netbsd:__aeabi_uldivmod
0x9ac8ddbc: netbsd:vfs_suspend+0x1b8
0x9ac8dddc: netbsd:vrevoke_suspend_next+0x3c
0x9ac8de14: netbsd:vrevoke+0xc4
0x9ac8de24: netbsd:genfs_revoke+0x20
0x9ac8de4c: netbsd:VOP_REVOKE+0x40
0x9ac8df14: netbsd:dorevoke+0x94
0x9ac8df34: netbsd:sys_revoke+0x44
0x9ac8dfac: netbsd:syscall+0x12c
cpu1: End traceback...
Undefined instruction 0xe7ffffff in kernel at 0x80023534 (LR 0x80265358 SP 0x9ac
8dd58)
Stopped in pid 621.1 (getty) at netbsd:cpu_Debugger:    und     0xe7ffffff
db{1}>

This is a -current kernel from sources updated about an hour ago,
userland is a couple of days old.