/Kill command doesn't work with relative coords

There are a few commands that use the relative coordinates. One command that doesn't appear to work with a relative command is /kill.

Example:
/kill @e[x=_10,y=,z=~,r=20,type=Item]

This should remove any dropped items in front of the command block however the "~" (relative) command does not function:

"The entity UUID provided is in an invalid format"