This affects only py3k, but apparently tkinter has been in this
situation for more than 9 months now.
I see these lines at _tkinter.c:

const char *s = Tcl_GetStringResult(self->interp);
const char *p = s;

res = PyUnicode_FromStringAndSize(s, (int)(p-s));

and I was wondering how could res not end up being an empty string
always ? Then I did some quick tests here and the return is always an
empty string, when wantobjects is set to False. Maybe the second line
should be:

const char *p = strchr(s, '\0');

I've attached this correction. Not sure if it is the best way to solve
the problem.