Can't invoke file picker in riverwm?

Hey everyone, I'm currently trying out river wm. For some reason, actions that should invoke the file picker dialog (like uploading a file to a webpage) fail to bring up the dialog and just do nothing.

I've tried this in firefox and chromium to no avail in river, though they both work as expected in other window managers (I've tested sway and labwc). Anyone run into this issue before?