Spreadplayers cannot use negative tilde value for x or z

When using spreadplayers, the tilde works for x and z when the tilde value is positive, but when it is negative, the spreadplayer command triggers an error, and nothing happens within the game.

I am trying to use spreadplayers to create a specialized "portal" structure that uses a repeating command block to send all players who enter the portal to an overworld surface location relative to the portal, in not-yet-generated random landscape. For this reason, the surface feature of spreadplayers is very useful, so that players don't appear inside of a stone wall and suffocate. However, since only positive tilde values are currently possible, this restricts the possible directions I can make these spreadplayers teleports go.

I hope you can add in recognition of negative tilde values to the spreadplayers command. Thank you!