Setting created/modified dates from markdown front matter

I can’t get that search qualifier you suggested to work either. For example, consider the following line of JavaScript:

const sel = app.search('kind:markdown', {in: app.databases['Personal']});

That gives me “Error -50: Parameter error.” It doesn’t seem to matter which database name I give it, it fails. If I simply remove the second argument, then I get all the markdown documents as expected, so it’s clearly the second parameter I’ve got wrong. Any suggestions?