The Sensex and Nifty ended flat after hitting a record high earlier as caution set in ahead of gross domestic product data later in the day, but still posted a 4.1 per cent gain for May, its best monthly performance in a year.The 30-share BSE index Sensex ended down by 13.6 points or 0.04 per cent at 31,145.80 and the 50-share NSE index Nifty closed lower by 3.3 points or 0.03 per cent at 9,621.25.