SA software says I owe tax even though UK earnings are under the single person allowance

Any help I can get would be very welcome. As a UK citizen living abroad, I have to file my Self Assessment return using third-party software. I've entered small UK payments (pension and interest) totalling less than £4,000. After filling in the residency section and claiming double taxation treaty relief (I'm in the USA and have included this UK income on my US tax return), it says I owe £450 in tax. Does that sound right? I had assumed that I would not owe anything because of the UK allowance.

Hi Ewan,
As a UK citizen, you are entitled to claim personal allowances in your tax return, no matter where in the world you reside.
It would appear that you have not done this.
If your UK pension is from a private pension, then it is not taxable in the UK (it is in the USA).
You should download and complete the DT individual form here declaring your pension in it:
Double Taxation: UK-USA (SI 2002 number 2848) (form US-Individual 2002)
You send the completed, signed and dated form to the IRS and for a fee, they will validate it and return it to HMRC.
HMRC can then take the validated form as confirmation of your resident in the USA and your tax liability there.
HMRC can then advise your pension provider to stop deducting tax.
If you have no other sources of UK income besides your pension and bank interest and the bank interest is below £10000, you do not need to complete tax returns.
You should  contact our self assessment helpline on:
(outside the UK) +44 161 931 9070 - Monday to Friday 8am to 6pm Closed on weekends and bank holidays or webchat with my self assessment colleagues here:
Self Assessment: general enquiries
To amend your tax return and advise them of your UK sources of income, to ensure a tax return is not required.
Thank you