I had the same message when trying to sort a file which contained a line with an invalid unicode character. My personal settings contained LC_COLLATE=en_US.UTF-8 (set|grep LC) When I either removed the invalid line or unset LC_COLLATE, the error was gone.
I consider this a bug in sort (My system: SunOS 5.8 Generic_Virtual sun4v sparc SUNW,SPARC-Enterprise-T5120) -- This message posted from opensolaris.org
