Capacitors in series

I needed a capacitor that can withstand over 400V. But what I had were 250V capacitors. So I put two identical capacitors in series. I monitored the voltages applied across each of the capacitors. There was surprisingly amount of imbalance, and progressively getting worse. One of the capacitors took up the larger share of the voltage. I placed 150K resistors across each of the capacitors, about 1mA and 0.5W dissipation. That was enough to restore the balance.