What is the test for striking testimony after a witness refuses to answer questions during cross-examination?

We review for abuse of discretion the district court's refusal to strike testimony after the witness refuses to answer questions during cross-examination. United States v. Montgomery, 998 F.2d 1468, 1478 (9th Cir.1993). The district court has "wide discretion" in making this determination. United States v. Seifert, 648 F.2d 557, 562 (9th Cir.1980) (quotations and citation omitted). The district court abuses its discretion by refusing to strike the witness's testimony "if invocation of the privilege blocks inquiry into matters which are 'direct' and are not merely 'collateral.' " Id. at: 561. Even if the district court errs by refusing to strike the testimony,. however, we review for harmless error. United States v. Williams, 626 F.2d 697, 702 (9th Cir.1980).