You get 0 on 6b if you didn't answer the Foreign question right. Or if the amount on 6b is too low and you are married you answered it wrong for one spouse.
There is a new question this year asking if you lived in a foreign place. People have been answering it wrong or skipping it. Go back through the Social Security entries and check, check for each spouse if married. If the IRS adjusted your return you do not need to amend.