bug in std.algorithm.sort ??

[A.M.]

import std.string;
import std.array;
import std.algorithm;

void main()
{
        string[] sa = ["TOM","JOE","TOM4","TOM12","TOM6"];
        sort!(comp)(sa);
}
bool comp(string x, string y)
{
        string[] s = [x,y];
        return isSorted(s);
}

this code compiles find, but when run gives the following error:

core.exception.asserter...@./../../src/phobos/std/array.d(210): Attempting to 
fetch the front of an empty array

I'm using dmd2.030
same happens with dmd2.029

is that a bug?