Error when using native system SSH on Windows 11

I installed and set up Git using the native OpenSSH implementation included with Windows, rather than the Git bash version. SourceTree is set to use OpenSSH as the SSH client, but every time I launch SourceTree it displays a popup error saying:

'ssh-agent' failed with code -1: System.NullReferenceException: Object reference not set to an instance of an object.

I can close the error and everything appears to work as expected in SourceTree (i.e. I can pull, fetch, push, etc.), but it's very annoying that this error occurs every time I open the app.