What is the test for establishing that a defendant's conviction may not stand?

It is a "bedrock, 'axiomatic and elementary' [constitutional] principle," that a defendant's conviction may not stand if it is based on an evidentiary presumption that essentially "reliev[es] the State of its burden of persuasion beyond a reasonable doubt of every essential element of a crime." Francis v. Franklin, 471 U.S. 307, 313, 105 S.Ct. 1965, 1970, 85 L.Ed.2d 344 (1985); see In re Winship, 397 U.S. 358, 363, 90 S.Ct. 1068, 1072, 25 L.Ed.2d 368 (1970). Collins claims that the district court established such an evidentiary presumption by failing to instruct on willfulness. In opposition, the government contends that the district court's omission was merely harmless error.