In Illinois Bell Telephone Co. v. Allphin, 95 Ill. App. 3d 115, 124, 419 N.E.2d 1188, 1196-97, 50 Ill. Dec. 739 (1981), the appellate court found that the Department of Revenue's regulations at the pertinent time stated that interstate revenues were not subject to a particular tax and that Illinois Bell had accordingly excluded those revenues from its tax computation. Allphin, 95 Ill. App. 3d at 124, 419 N.E.2d at 1196-97. Holding that the Department could not retroactively assess the tax, the appellate court held that the Department attempted "to the absolute detriment of the taxpayer who relies on the Department's rules and regulations, to totally ignore its own written rules and regulations and retroactively assess a tax when clearly none was due" under the rules in effect during the relevant tax period. Allphin, 95 Ill. App. 3d at 126, 419 N.E.2d at 1198. Therefore, in Allphin, the state's Department of Revenue attempted to disavow its own regulations, and Illinois courts prohibited the Department from retroactively applying newly revised rules when the complaining party had relied on the original rules in effect at the time. After Allphin was decided, the Illinois legislature added the final sentence to section 5-10(c) to reflect that principle. 83rd Ill. Gen. Assem., Senate Proceedings, June 21, 1984, at 91 (statements of Senator Bloom).