In case the board of park commissioners of any city shall levy any special assessment, which assessment is or may be informal, illegal, irregular or void for the want of sufficient authority to make or levy the same for any cause whatsoever, such board of park commissioners may at any time reapportion the cost of the improvement and relevy assessments therefor, or may relevy any such special assessment or assessments in the manner provided, and against the property liable for assessment for the improvement at the time of the making thereof: Provided, That no such reapportionment or relevy shall be made in any case in which there has not been a substantial performance of the contract.