the ping6 command returns an error for valid command lines:

EXAMPLES:

[root@ultra1 iputils-20000121]# ping6 -I eth0 hank
ping6: socket: Invalid argument
[root@ultra1 iputils-20000121]# ping6 -I eth0
ping6: socket: Invalid argument
[root@ultra1 iputils-20000121]#

from /usr/doc/iputils-20000121/README:
------------------------------
* ping/ping6

Usage: ping [ as usually ]

	Changed options: -I <device name> may be used to set output interface.

	New options:
	-T [ping only]:  send timestamp IP option.
		-T tsonly	--- only timesamps
	        -T tsandaddr	--- timestamps & addresses
		-T tsprespec host1 [host2 [host3 [host 4]]]
				--- timestamp prespecified hops

	-P <traffic class> [ping6 only] sets traffic class bits.
	-F <flow label> [ping6 only] sets flow label. -F 0 generates
	    random label.
	-w TIMEOUT  exit after this time. If no replies received, exit
	            with error code.

	Improvements:
	- it understands icmp error messages and reacts to them correctly.
	- it checks icmp checksum and reacts to corrupted packets correctly.
	- it correctly pings linux loopback.
	etc.
---------------------------------