If upon a canvass of the election it is found that a majority of all votes cast on the proposition at the election were cast in favor of the annexation, the proposition and all of the terms and conditions of the agreement of annexation shall be deemed carried and approved by the voters; except that if the terms and conditions of the agreement of annexation provide for the assumption of any indebtedness of the district by any city proposed to be annexed, the proposition of annexation shall not be deemed carried unless approved by the vote of two-thirds of all the voters voting on the proposition at the election.