65 ILCS 5/9-2-137
The board of local improvements before the crediting of the excess as provided for in Section 9-2-114, shall estimate an amount deemed sufficient to make up any probable deficiency of interest, in the event that from any cause, collections of interest may prove insufficient to meet the interest to be paid on the bonds until they mature as hereinbefore provided. This estimated amount shall be deducted out of the installments as an item of expense before crediting rebates of excess as directed in this Division 2 and shall be used for no other purpose than to make up such a deficiency until the bonds are fully paid, both principal and interest. Any balance remaining of this estimated amount after the principal and interest of the bonds are fully paid may be used to reimburse the corporate fund for any advances made from this fund on account of costs of the special assessment or special tax or other expenses of the improvement for which the special assessment or special tax is levied.
However, in municipalities having a population of 500,000 or more, no deduction of this estimated amount out of the installments shall be made where the ordinance providing for the assessment provided that a certain sum not to exceed 5% of the amount of that special assessment or special tax shall be applied as provided in Sections 9-2-138 and 9-2-139, or in case such a municipality, at any time before the crediting of such excess, shall annually appropriate or set aside a fund sufficient in amount to meet all estimated deficiencies in interest which may arise during the year for which the fund is provided.
65 ILCS 5/9-2-137