Duck with Orange Gastrique

The instructions (and accompanying pictures) for cooking the duck are excellent and result in a really well-cooked duck breast. I had some problems with the gastrique -- it is easy enough to do (blanch some orange zest and add it and some red wine vinegar to a half cup of carmelized sugar). But my gastrique had a bitter undertone and it was not because I had captured too much (or any) pith with the zest. So ... worth a second try to see if it was a fluke.