In his trial for wire fraud and money laundering, appellant was prohibited from fully cross- examining his brother, a key witness who had been granted immunity, about certain hearsay statements. Since the trial court’s rulings unnecessarily limited relevant, probative, and crucial evidence concerning the credibility of a key government witness, the appellate court here reversed. Since the case turned on a credibility contest between the two brothers, the trial court’s ruling precluded appellant from making his case, so the error was not harmless.