Most likely it's a locale problem. Try "sort --debug" and try setting LC_ALL='C' in your environment.
- bug#12907: Possible Bug in sort core u... Coffey, Terrence (Terrence) **CTR**
- bug#12907: Possible Bug in sort c... Paul Eggert
- bug#12907: Possible Bug in sort c... Bob Proulx
- bug#12907: Possible Bug in so... Coffey, Terrence (Terrence) **CTR**
- bug#12907: Possible Bug i... Pádraig Brady
- bug#12907: Possible B... Coffey, Terrence (Terrence) **CTR**
- bug#12907: Possible Bug in sort c... Eric Blake