Issue with Sudo for Non Standard Root user

Hi All,
I have replaced root with rootadmin user while building openwrt,
All things are working perfectly,
Now i want to use sudo for non-root user.
i have setted up sudoers file as below;
But whenever i try to do "sudo -i" it gives error as "sudo: unknown user: root"

rootadmin ALL=(ALL) ALL


## Read drop-in files from /etc/sudoers.d
@includedir /etc/sudoers.d

You have to patch sudo

Since you are building from source, you could/should build the busybox setpriv applet and use that instead of sudo to switch from UID 0 to any other.

Ceterum censeo should have been accepted.