How to sync to local clock using chrony

Issue

chrony does not sync to local clock.

The local server "127.127.1.0" is added to /etc/chrony.conf.

server 127.127.1.0
allow 127.0.0.0/8
local stratum 10

It synchronized with the local server after the daemon started. 'chronyc sources' command gives the following result.

210 Number of sources = 1
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
^* 127.127.1.0                  15   6   377    42  -4471ns[  -13us] +/-  204us

However, the error "Can't synchronise: no selectable sources" appears in /var/log/messages and the local sever is no longer selected as a selectable NTP server.

210 Number of sources = 1
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
^? 127.127.1.0                  15   4     0   150  -6275ns[  -11us] +/-   80us