Former Mayor Ray Nagin, now serving a 10-year prison sentence after his conviction last year on corruption charges, is asking an appellate court to vacate the verdict against him on the grounds that the jury was not instructed properly on the federal law regarding “honest services fraud.”

In short, the appeal argues that a 2010 U.S. Supreme Court ruling in U.S. v. Skilling narrowed the scope of that law, which is often used in prosecuting political corruption and which has come under fire over the years because of its vagueness. The high court’s ruling in the Skilling case limited its future application to “bribes and kickbacks,” the appeal notes.

Nagin’s case did not meet that bar, according to the appeal, because most or all of the official actions Nagin took as mayor on behalf of people who were paying him — such as businessmen Frank Fradella and Rodney Williams — were actions he likely would have taken anyway. For instance, as mayor, Nagin signed city contracts with companies owned by Fradella and Williams, but he was required to sign those contracts under the city charter, the appeal says.

But U.S. District Judge Ginger Berrigan told the jury that considered Nagin’s case that “it is not a defense to claim that a public official would have lawfully performed the official action in question even without having accepted a thing of value,” according to the appeal.

It took jurors just a few hours to find the former mayor guilty on 20 of 21 counts.

Although Robert Jenkins, who represented Nagin at the trial, did not object to Berrigan’s instructions, they were improper under the Skilling ruling, according to the appeal, which was filed to the 5th U.S. Circuit Court of Appeals by Assistant Federal Public Defender Jordan Siverd on Tuesday. Siverd’s appeal says the government was required to prove that Nagin took specific actions in exchange for money, and that it failed to do so.

The claim that the honest-services statute was misapplied in Nagin’s case forms the centerpiece of the appeal, and is the basis for Siverd’s argument that nine of the 20 counts on which Nagin was convicted should be thrown out.

But the appeal also says the other 11 counts Nagin was convicted of should be thrown out as well, in part because of the “prejudicial spillover” from the counts involving honest-services fraud. Four of those 11 counts were bribery charges — perhaps undercutting the appeal’s argument that the honest-services law was not applicable to Nagin.

The appeal also says Berrigan erred in ordering Nagin to forfeit $501,201, saying the law authorizes forfeiture of property but does not allow for “personal money judgments.” In addition, it says, the amount was calculated incorrectly.

Nagin had filed notice in July saying he intended to appeal.

Jenkins withdrew from the case shortly after the trial, saying Nagin could no longer afford to pay him. Berrigan ruled in September that the former mayor was indigent and eligible for a public defender after Nagin reported having only $23.65 in his checking account.

Nagin began his 10-year prison term at a federal prison camp in Texarkana, Texas, that same week.

Follow Gordon Russell on Twitter, @gordonrussell1.