Here's what the IRS says:

"We calculate the Failure to File Penalty based on how late you file your tax return and the amount of unpaid tax as of the original payment due date (not the extension due date). Unpaid tax is the total tax required to be shown on your return minus amounts paid through withholding, estimated tax payments and allowed refundable credits."

https://www.irs.gov/payments/failure-to-file-penalty

It seems that there would be a late filing penalty due + interest.