Can't boot 5.99.55 on amd64 machine

[Hugo Silva <hugo%barafranca.com@localhost>]

Dear reader,

It may be a trivial mistake on my side, as it's been some time since I
last installed NetBSD over a serial console.

Here' what I'm seeing:

Pressa<Ctrl-E>6for9Remote Access Setup within 5 sec......

>> NetBSD/x86 BIOS Boot, Revision 5.9 (from NetBSD 5.99.55)
>> Memory: 635/1042752 k
> load miniroot.kmod
> load /miniroot.kmod
> load /stand/amd64/5.99.55/modules/miniroot/miniroot.kmod
> boot -m
11998648+622216+586232 [808416+533371]=0xee1480
WARNING: couldn't open miniroot.kmod
(/stand/amd64/5.99.55/modules/miniroot.kmod/miniroot.kmod.kmod)
WARNING: couldn't open /miniroot.kmod
Loading /stand/amd64/5.99.55/modules/miniroot/miniroot.kmod  FAILED
Loading /stand/amd64/5.99.55/modules/cd9660/cd9660.kmod
WARNING: 2 modules failed to load
Copyright (c) 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
    2006, 2007, 2008, 2009, 2010, 2011
    The NetBSD Foundation, Inc.  All rights reserved.
Copyright (c) 1982, 1986, 1989, 1991, 1993
    The Regents of the University of California.  All rights reserved.

NetBSD 5.99.55 (GENERIC) #0: Tue Aug  9 21:27:02 UTC 2011
        
builds%b7.netbsd.org@localhost:/home/builds/ab/HEAD/amd64/201108091930Z-obj/home/builds/ab/HEAD/src/sys/arch/amd64/compile/GENERIC
(...)

At this point, most unusually, it asks me for the root device:
boot device: <unknown>
root device: cd0a
dump device (default cd0b):
file system (default generic):
root on cd0a dumps on cd0b
root file system type: cd9660
warning: no /dev/console
init path (default /sbin/init):
init: trying /sbin/init
Created tmpfs /dev (1523712 byte, 2944 inodes)
panic: kernel diagnostic assertion "cnp->cn_nameptr[0] != '/'" failed:
file "/home/builds/ab/HEAD/src/sys/kern/vfs_lookup.c", line 1139
fatal breakpoint trap in supervisor mode
trap type 1 code 0 rip ffffffff80251bb5 cs 8 rflags 246 cr2
7f7ffffe46d8 cpl 0 rsp ffff80004a333910
Stopped in pid 25.1 (ls) at     netbsd:breakpoint+0x5:  leave

Since the server is halfway across the globe, I'm unable to test with a
more recent ISO (this one is roughly 2 weeks old) right away.

I suspect it is a bug, most likely fixed in the mean time. But it could
be me missing something very obvious.

Thoughts?

Thanks for reading.
Best regards,

Hugo