exclusive sleep mutex ifnet r = 0 (0xfffffc00006dca90) locked @sys/nfsclient/bootp_subr.c:1692

I'm getting the following when I try and netboot the alpha package
machines (note also the missing space in the diagnostic message):

malloc() of "4096" with the following non-sleepablelocks held:
exclusive sleep mutex ifnet r = 0 (0xfffffc00006dca90) locked _at_ /a/asami/portbuild/alpha/src-client/sys/nfsclient/bootp_subr.c:1692
witness_warn
Stopped at      Debugger+0x38:  zapnot  v0,#0xf,v0      <v0=0x0>
db> trace
Debugger() at Debugger+0x38
witness_warn() at witness_warn+0x284
uma_zalloc_arg() at uma_zalloc_arg+0xd0
malloc() at malloc+0x11c
allocifctx() at allocifctx+0x30
bootpc_init() at bootpc_init+0x144
nfs_mountroot() at nfs_mountroot+0x30
nfs_mount() at nfs_mount+0x34
vfs_mountroot_try() at vfs_mountroot_try+0x250
vfs_mountroot() at vfs_mountroot+0xb4
start_init() at start_init+0x80
fork_exit() at fork_exit+0x100
exception_return() at exception_return
--- root of call graph ---
db>