[gcc 3.2.3/Linux] std::wfilebuf extracts more than 1 character from a 1 byte file

#include <cassert>
#include <fstream>
#include <iostream>

int main ()
{
    {
        std::filebuf fb;
        fb.open ("file", std::ios::out);
        fb.sputc ('a');
    }

    std::wfilebuf fb;
    fb.pubimbue (std::locale ("en_US.UTF-8"));
    fb.open ("file", std::ios::in);

    const int c[] = { fb.sbumpc (), fb.sgetc () };
    std::cout << c [0] << ' ' << c [1] << std::endl;

    assert (L'a' == c [0]);
    assert (std::wfilebuf::traits_type::eof () == c [1]);
}

test: test.cpp:21: int main(): Assertion `std::wfilebuf::traits_type::eof () == c [1]' failed.
Aborted