NEW YORK - Irwin Lipkin, one of Bernard Madoff's longest-serving employees, was sentenced to six months in prison on Wednesday for falsifying records that helped the imprisoned fraudster carry out his multibillion-dollar Ponzi scheme.The sentencing before US District Judge Laura Taylor Swain in Manhattan marked the end of the criminal prosecutions stemming from the fraud, more than six years after Madoff's arrest sent shockwaves through Wall Street.Lipkin, 77, was the Madoff firm's controller from 1964 to 1998. He was the last of 15 defendants who either pleaded guilty or were convicted at trial to be sentenced.Madoff, also 77, is serving a 150-year prison term after pleading guilty in 2009 to masterminding the scheme, estimated to have cost investors $17 billion in principal.Lipkin pleaded guilty in 2012 but, like other defendants, said he was unaware Madoff was running a Ponzi scheme."Smarter people than myself were taken in by him," Lipkin, who was confined to a wheelchair, told Swain on Wednesday.But Assistant US Attorney David Abramowicz said Lipkin, and others, helped Madoff perpetuate his scheme through their crimes, even if they didn't know the entire truth."The lesson we've seen in this parade of guilty defendants is: Bernard Madoff didn't do this alone," he told Swain. "He needed help, and he found it in people like Irwin Lipkin."Swain said she would have imposed a far longer sentence if not for Lipkin's age and poor health.