The Sensex at the Bombay Stock Exchange declined 64 points, or 0.3 percent, to 19,856, today, ahead of the expiry of the monthly derivatives contracts, and amid weak Asian markets. The Nifty at the National Stock Exchange fell 19 points, or 0.3 percent, to 5,874.Stock markets in Japan, China, South Korea and Singapore declined between 0.1 percent and 0.8 percent.