Lawyers at Cahill Gordon & Reindel knocked out a proposed investor class action against Deutsche Bank AG on Tuesday, persuading a federal judge in Manhattan that the plaintiffs relied on an unreliable expert witness to support their bid for class certification.

U.S. District Judge Katherine Forrest didn’t just find flaws in the methodology employed by the expert, Michael Marek, who was tasked by the plaintiffs with establishing that Deutsche Bank’s shares traded in an efficient market. She also took issue with Marek’s credibility, writing that he has “not been specially trained by academics in the field; he has not written articles, taught any courses or conducted any relevant research.” Deutsche Bank, in contrast, brought in Stanford Law School’s Joseph Grundfest and Paul Gompers of Harvard Business School to rebut Marek’s findings.

Plaintiffs lawyers at Robbins Geller Rudman & Dowd had urged the judge to certify a proposed class of individuals and entities who purchased Deutsche Bank shares between January 2007 and January 2009, alleging that the bank misled investors about its holdings of risky mortgage-backed securities. Class cert is rarely a huge obstacle for plaintiffs in securities cases, in large part because they can invoke the so-called fraud-on-the-market presumption of reliance. That doctrine holds that investors in an efficient market rely on all available information—including a defendant’s public misrepresentations—in choosing to purchase shares. For the doctrine to kick in, plaintiffs lawyers simply need to show that the shares were trading in an efficient market during the relevant period.

That hurdle proved too much for Robbins Geller. The firm retained Marek, one of its go-to expert witnesses, who testified about a study he conducted that supposedly showed that there was an efficient market for Deutsche Bank global registered shares (GSRs). According to Marek’s study, market efficiency existed because “when important, unexpected news about [Deutsche Bank] was released to the market, the price of its GRSs moved in a directionally appropriate way.”

Deutsche Bank’s defense lawyers at Cahill, led by partner Charles Gilman, turned to Grundfest and Gompers to poke holes in Marek’s methodology. And poke holes they did.

Siding with Deutsche Bank’s defense team, Forrest found “significant flaws” in Marek’s study. The judge wrote, for instance, that “Marek has not studied the efficiency of the German market.” According to the judge, “the German market was the primary price driver for GSRs trading in the U.S.”

Forrest also raised doubts about Marek’s credentials—and flat out called him “unqualified” to opine on market efficiency. The judge wrote that “the only writing that Marek has ever done on market efficiency is in connection with declarations for Robbins Geller and other similarly situated law firms in connection with federal court litigation.” She also noted that he spent the first ten years of his professional career as an assistant to John Torkelsen, a former expert witness in securities cases who pleaded guilty to perjury in 2008.

Back in March, Forrest refused to certify a class action against China Automotive Systems Inc. on similar grounds.

Robbins Geller partner John Grant did not immediately return a call seeking comment.