If any land is not charged with its portion of any assessment or if the assessment on any land is adjudged invalid by any court of competent jurisdiction, that land shall at any subsequent assessment hearing be assessed, in addition to the then current assessment, by the board of supervisors in a sum which bears the same proportion to the total amount of the former assessment as its then assessed valuation bears to the total amount of the assessed valuation placed on all the land at the time the former assessment was levied.