Overfflow in Assert error messages

[Adnan]

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 at source/binary_search.d(33): 
18446744073709551615 != 1

What's causing this underflow?

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