Ahhh... That's where the 5th amendments been hiding :-D
Sullivan and Garner are viewed by some legal scholars as standing, in tandem, for the proposition that on a required Federal income tax return a taxpayer would probably have to report the amount of the illegal income, but might validly claim the privilege by labeling the item "Fifth Amendment" (instead of "illegal gambling income," "illegal drug sales," etc.)
I don't know how current those citations are. But the underlying claim is that a tax return filed reporting illegal income can't be used against a defendent in a criminal case.
The Court stated that "if a witness under compulsion to testify makes disclosures instead of claiming the privilege, the Government has not 'compelled' him to incriminate himself."
Sounds like he should have had a better defense lawyer and or tax lawyer.