Contracts to buy previously owned U.S. homes unexpectedly dropped in April to a four-month low, undermining some of the recent optimism that the housing sector was beginning to improve. The National Association of Realtors (NAR) reported Wednesday its pending home sales index, based on contracts signed in April fell 5.5 percent to 95.5, its lowest level since December. Economists had expected pending sales, which lead existing-home sales by one or two months, to rise 0.1 percent after a 4.1 percent gain in March. A reading of 100 is considered healthy. One year ago, the NAR index stood at 83.5. The housing market has been the weakest sector of the U.S. economy as it recovers from the 2007-2009 recession, but many economists believe the sector actually will add to economic growth in 2012 for the first time since 2005. After a debt-fueled housing bubble, prices have fallen by about one-third since 2006, according to some measures, and the housing market continues to suffer from an oversupply of unsold properties.