I agree with macuser_22. Since you have already filed your 2017 tax return, I would wait until you receive the code JP 2018 Form 1099-R to amend your 2017 tax return and pay the tax on the $18 of earnings. The IRS won't bill for any interest on the taxes resulting on the $18 of income; the interest will likely be less then 15 cents.
(The answer of mine referenced above applies to a different situation where the return of contribution had already been reported. It's not relevant here.)
