{first_name} field generates first and last name

I’m noticing that my renewal reminder emails are showing the full first and last name, even though I’m only using the {first_name} field in my email template. Am I doing something wrong? Is there a different field I should be using to only show the first name?
This is the code I’m using:
<p>Hi {first_name}, <br />This is just to let you know that your membership subscription is set to renew on {next_payment_date}.</p>