Overfflow in Assert error messages

[Adnan via Digitalmars-d-learn]

I am debugging my simple binary search (I still am):

module binary_search;
debug {
    static import std;
}

int indexOf(T)(const T[] list, const T key) {
    ulong lo = 0;
    ulong hi = list.length - 1;
    while (hi > lo) {
        const ulong mid = lo + (hi - lo) / 2;
        if (list[mid] > key)
            hi = mid - 1;
        else if (list[mid] < key)
            lo = mid + 1;
        else {
            std.writeln("----Returning ", mid, "----"); // says 
its returning 0
            return cast(int) mid;
        }
    }
    return -1;
}

unittest {
    scope (success)
        std.writeln("binary_search.indexOf -- ok");

    int[] arr;
    foreach (i; 0 .. 101)
        arr ~= i;
    assert(arr.length > 1);
    foreach (idx, i; arr)
        assert(indexOf(arr, i) == idx);
}


However my test fails saying something like:
source/binary_search.d(33): [unittest] 18446744073709551615 != 1
core.exception.AssertError@source/binary_search.d(33): 
18446744073709551615 != 1

What's causing this underflow?

I am using "dflags": ["-checkaction=context"] in my dub 
configuration file.