I withdrew 2018 my RMD on 12/24/2018, filed my return 4/5/2019, but I received a tax penalty on 4/29/2019 for "Failure-to-pay proper estimated tax" for 2018. I had no other income for 2018 other than a monthly pension of $24.93 (total of $299.16, which I added to the RMD). Am I supposed to pay tax on income I haven't received yet (the RMD)? The billing summary (attached) shows tax I owed equaled exactly my "payments and credits" to the penny. But added $223.61 for "failure to pay proper estimated tax penalty." Never explained why there was a penalty.