Extra Keyboard Shortcuts for Porportional Editing?

It's nice to enable/disable proportional editing with O, but I'd like to switch between falloff settings with a keyboard shortcut. If I open the the menu and right click on an option, the "Assign Shortcut" option is there, but blender doesn't seem to be able to differentiate between the different modes if I assign different shortcuts:

Is there any way to get it to work?

You can search the shortcut in preference with keyword falloff: