The IRS provides a little more information on the subject on this FAQ: Will I be charged interest and penalties for filing and paying my taxes late?:
If you did not pay your tax on time, you will generally have to pay a late-payment penalty, which is also called a failure to pay penalty.
- You will not have to pay the penalty if you can show reasonable cause for the failure to pay on time.
Some guidance on what constitutes "reasonable cause" is found on the IRS page Penalty Relief Due to Reasonable Cause:
The IRS will consider any sound reason for failing to file a tax return, make a deposit, or pay tax when due. Sound reasons, if established, include:
- Fire, casualty, natural disaster or other disturbances
- Inability to obtain records
- Death, serious illness, incapacitation or unavoidable absence of the taxpayer or a member of the taxpayer’s immediate family
- Other reason which establishes that you used all ordinary business care and prudence to meet your Federal tax obligations but were nevertheless unable to do so
Note: A lack of funds, in and of itself, is not reasonable cause for failure to file or pay on time. However, the reasons for the lack of funds may meet reasonable cause criteria for the failure-to-pay penalty.