inconsistent invocation between touch and core

For touch (and ubuntu-emulator) I can do:
$ ubuntu-device-flash touch --channel=ubuntu-touch/rc-proposed/ubuntu
...
2015/05/22 12:47:41 Flashing version 139 from ubuntu-touch/rc-proposed/ubuntu channel and server https://system-image.ubuntu.com to device mako

ubuntu-emulator is consistent too:
$ sudo ubuntu-emulator create --channel=ubuntu-touch/rc-proposed/ubuntu --arch=i386 rc-proposed.x86
Creating "rc-proposed.x86" from ubuntu-touch/rc-proposed/ubuntu revision 139

However, with core I get:
$ sudo ubuntu-device-flash core --channel=ubuntu-core/15.04/edge --size=8 --enable-ssh --output=15.04-alpha.img
the required argument `Release` was not provided

What does work is:
$ sudo ubuntu-device-flash core --channel=edge --size=8 --enable-ssh --output=15.04-alpha.img 15.04
...
Summary:
 Output: 15.04-alpha.img
 Architecture: amd64
 Channel: edge
 Version: 64