The couple, both over 80 years' old, didn't file tax return last year, because their income was only Social Security Benefits income. But this time, because they closed their "brokerage" accounts by selling all the securities, they received Form 1099-B and Form 1099-DIV, which included "Foreign taxes"

I thought that they need to file the return this time, and entered Schedule D (Form 8949) and Form 1116.  Then when I looked at Form 1040, there was no tax computed.  

I am perplexed with this result.  Taxable income is about $25K, after Standard deduction.  But there is no tax.  Was I wrong when I thought that the couple has to file the return?

Maybe my understanding was wrong?