I think you have a data entry error. It seems MOST unlikely that the Illinois property tax credit - which is only 5% of the property tax paid - would wipe out the entire tax due on a taxpayer's income. I would start by double-checking the entries on your Schedule ICR.
As @Mike9241 indicated, if there were a TT programming glitch, tens of thousands of Illinois TT users would have been affected, which didn't happen.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
