The 1988 trial relied on testimony from Holocaust deniers David Irving and Fred A. Leuchter, a self-taught execution technician. Leuchter's testimony as an expert witness was accepted by the court, but his accompanying Leuchter report was excluded, based on his lack of engineering credentials. In 1985, key expert testimony against Zündel's alleged Holocaust denial was provided at great lengths by Holocaust historian Raul Hilberg, who refused to testify at Zündel's 1988 trial. Zündel was convicted in 1988 and sentenced to 15 months imprisonment by an Ontario court; however, in 1992 in "R v Zundel" his conviction was overturned by the Supreme Court of Canada when the law under which he had been charged, "reporting false news", was ruled unconstitutional.