A voter may not vote for a person whose name is not on the ballot. Votes cast for a person whose name is not on the ballot may not be counted, but writing in a candidate's name does not invalidate the entire ballot.
Alaska Legal Code
A voter may not vote for a person whose name is not on the ballot. Votes cast for a person whose name is not on the ballot may not be counted, but writing in a candidate's name does not invalidate the entire ballot.