What is the test for establishing that a harmless error in admitting to a guilty plea is harmless?

Applying this framework to the present case, we conclude that the Government has not met its burden of establishing harmless error. See United States v. Velarde-Gomez , 269 F.3d 1023, 1035 (9th Cir. 2001) (en banc) (The burden of proving a constitutional error harmless beyond a reasonable doubt rests upon the government.). The Government asks and answers the wrong question when it argues that admission of the Car Phone evidence was harmless because it was immaterial to Lustig's conviction. The relevant inquiry is whether the erroneous admission of the Car Phone evidence was immaterial to Lustig's decision to enter a guilty plea. Given the dearth of factual clarity in the record as to Lustig's plea considerations, and indeed as to what evidence, exactly, was derived from the Car Phones, we cannot conclude beyond a reasonable doubt that the Car Phone evidence did not contribute to Lustig's decision to plead guilty.