It was a while ago, but IIRC you only owe taxes on the pre-tax contributions (which, in this case was $0). But I couldn't make a post-tax contribution directly to a Roth, just a post-tax contribution to a traditional, then convert...

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Some googling[1] implies my memory was mostly correct.

1: https://www.investopedia.com/roth-ira-conversion-rules-47704... See particularly the part about "backdoor