spreadplayers command doesn't accept relative coords for x and z parameters

Entering the spreadplayers command with relative (~) x and/or z coords causes a "NaN is not a valid number" error.

Ex:
/spreadplayers ~0 ~0 10 20 false @a

This would be very useful for modular design, allowing players to be spread out relative to a given player/command block location, without being tied to a specific x/z location.