Thank you for all the responses. This information is kind of disconcerting. The wheel in question is fairly high end, with Mavic 719 Disc rim and DT Swiss hub. It was built by Bicycle Wheel Warehouse. I assumed they would use the proper length spokes for building a wheel but now that nipples keep breaking, I'm beginning to wonder. This problem just started occurring about two years after I first put the wheel on the bike.The nipples break right where the flange of the nipple meets the rim. Could it be a case of uneven spoke tension? Although the wheel looks fairly true and round on the truing stand, I noticed some spokes are tighter than others. Thank you in advance for any advice.