geezermike, unless previously distributed, there must be basis from either contributions or conversions. If there is no basis regular contributions, then the only basis would be from Roth conversions and your Roth IRA contribution basis would be zero.
Since you are over age 59½ and have had a Roth IRA more than 5 years, distributions are qualified distributions, not reported on Form 8606, your basis (either contribution basis or conversion basis) is largely irrelevant and you don't need to enter it.
