How have the courts interpreted the burden of proof in the case of Flores?

Flores next contends that the government improperly (1) shifted the burden of proof by pointing out that she failed to produce evidence to corroborate her story about Juan the mechanic and (2) undermined the burden of proof by asking the jury to evaluate the government's very reasonable story and Flores's preposterous story in the same way. See United States v. Evanston, 651 F.3d 1080, 109192 (9th Cir.2011) (It is beyond comment that the government bears the burden of proving the defendant's guilt beyond a reasonable doubt at trial.). These statements were not improper.