Netifd troubles

I'm trying to implement nebula as a netifd-compatible service (so that, for example, I can use it as an interface in dropbear settings).

This is the file I have: https://github.com/stangri/source.openwrt.melmac.net/blob/85302ac1723408dec77521af74c9f21b52fb90da/nebula/files/nebula.proto

The interface does show up in ifconfig:

SG-105 in ~ # ifconfig nebula1
nebula1   Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
          inet addr:192.168.*.*  P-t-P:192.168.*.*  Mask:255.255.255.0
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1300  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:500
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

However, when I add this to /etc/config/dropbear and restart it, I get:

config dropbear
	option Port '2222'
	option Interface 'nebula1'
	option RootPasswordAuth 'off'
	option PasswordAuth 'off'
SG-105 in ~ # service dropbear restart
interface nebula1 has no physdev or physdev has no suitable ip

I'm guessing I'm missing something in the proto script, but I don't know what and/or how to debug it.

Please help.