The prosecution of Venezuela’s Oil Minister Tareck El Aissami for violating U.S. sanctions has run into another snag after a federal judge allowed one of his co-defendants to withdraw a guilty plea over allegations that U.S. attorneys withheld evidence in the case.

Federal prosecutors in Manhattan on Monday said they don’t object to Victor Mones withdrawing his guilty plea, but refused to concede they violated rules requiring them to turn over all exculpatory evidence to the defendant.