Are you running an older version?
When you say this doesn't happen on Ubuntu 12.10 I assume you mean an X86 build and not an ARM build, right?
In fact, I'll bet this was fixed with SVN rev 271 which had to do with compilers that default chars to unsigned (which I think ARM does, for some reason).
I couldn't resist trying immediately Current revision (280) works fine, 270 has the bug and 271 works fine too, so it seems r271 indeed fixed this.
A WavPack maintenance release is way past due....