Hope this helps.
The Fifth Amendment 'can be asserted in any proceeding, civil or criminal, administrative or judicial, investigatory or adjudicatory; and it protects against any disclosures which the witness reasonably believes could be used in a criminal prosecution or could lead to other evidence that might be so used.' Kastigar v. U.S., 406 U.S. 441, 44-45 ('72). A reasonable belief that information concerning income or assets might be used to establish criminal failure to file a tax return can support a claim of Fifth Amendment privilege. See U.S. v. Rendahl, 746 F.2d 553, 55-56 (9th Cir.'84).