How to set ipv4 default route

[Pete Zaitcev <zaitcev redhat com>]

Greetings:

Looks like I'm not done with user's questions yet. This time I'm failing
at something as basic as setting a default route. It looks like this:

[root localhost ~]# nmcli c mod eth0 ipv4.routes 'default 192.168.129.17'
Error: failed to modify ipv4.routes: invalid IPv4 route 'default'.
[root localhost ~]# nmcli c mod eth0 ipv4.routes '0.0.0.0/0 192.168.129.17'

** (process:2380): CRITICAL **: nm_ip4_route_set_prefix: assertion `prefix > 0' failed
Error: Failed to modify connection 'eth0': (32) ipv4.routes: 1. route is invalid
[root localhost ~]# 

[root localhost ~]# nmcli c mod eth0 ipv4.routes '0.0.0.0 192.168.129.17'
Error: Failed to modify connection 'eth0': (32) ipv4.routes: 1. route is invalid
 <===== this one has prefix /32, probably

Clearly I'm missing something too obvious!

What's interesting, the way to modify the ipv4.routes is not discussed
in any docs. Not in nmcli(1) certainly, but neither in nmcli-examples(5).
Still, NetworkManager.conf(5) contains this tantalizing hint:

           Remember that NetworkManager controls the default
           route, so because the interface is ignored, NetworkManager may
           assign the default route to some other interface.

So... Perhaps the default route is not even set in the connection properties.
But I cannot figure it out.

-- Pete