Of course you are right. Thank you for pointing this out. I misspoke, so to speak.
It was Arthur. There is actually some evidence he may have been born a few miles across the Canadian border, leaving his “natural born” status in question.
This issue was not kept secret very well. The opposition tried to use it against him in the election, but it never got traction with the electorate. This may have partly been because he was running for veep, and it’s harder to get people worked up over the veep’s qualifications.
There was another 19th century president whose mother was born in Canada and may never have gone through a formal naturalization process. Although in at least the early 19th century a woman’s citizenship automatically changed when she married a foreigner.