Remove IP from WHM

Hi guy,

My situation:

1) I create a reseller account and assign 2 new IP for nameserver use.
2) Customer register his nameserver.
3) Terminate the reseller account
4) Try to remove the IP from WHM and having this error massage from WHM
"sory,the nameserver IP xxx.xxx.xxx.xxx cannot be removing since it is still registered with an ICANN register"

Much pleasure if you guy can advice me how am i call back this 2 IP.

Thank you so much.

kelvin