IPv6 Neighbor Discovery Protocol on Cisco Router

Hi Rene and staff,
still studying ipv6 and testing ND cache, ipv6 states, NUD, etc …
and relative cisco commands
So i do a basic lab with 2 CiscoIOSv15.6(1)T-1


I set a static entry in the ND cache and i cannot delete it. Please look the above pic

I google the error message “Failed to delete ND entry”, but found nothing
Could you help me to understand ?
Regards