It's usually best to go the IRS publications:
IRS Publication 590
Additional Tax on Early Distributions
If you receive a distribution that is not a qualified distribution, you may have to pay the 10% additional tax on early distributions as explained in the following paragraphs.
Distributions of conversion and certain rollover contributions within 5-year period. If, within the 5-year period starting with the first day of your tax year in which you convert an amount from a traditional IRA or rollover an amount from a qualified retirement plan to a Roth IRA, you take a distribution from a Roth IRA, you may have to pay the 10% additional tax on early distributions. You generally must pay the 10% additional tax on any amount attributable to the part of the amount converted or rolled over (the conversion or rollover contribution) that you had to include in income (recapture amount). A separate 5-year period applies to each conversion and rollover. See Ordering Rules for Distributions , later, to determine the recapture amount, if any.
Note the part in bold (emphasis mine). If your conversion had no earnings, then there would be no additional tax.