35 ILCS 735/3-8

The penalties imposed under the provisions of Sections 3-3, 3-4, 3-5, and 3-7.5 of this Act shall not apply if the taxpayer shows that his failure to file a return or pay tax at the required time was due to reasonable cause. Reasonable cause shall be determined in each situation in accordance with the rules and regulations promulgated by the Department. A taxpayer may protest the imposition of a penalty under Section 3-3, 3-4, 3-5, or 3-7.5 on the basis of reasonable cause without protesting the underlying tax liability.