When I choose to create a "New Mapped Key" I can choose to use any of the buttons on my mouse for the "Hotkey". But after I create a step and make a WoW Macro and go to choose a "Key combination" I cannot choose any of my mouse buttons?

Is this something new that I missed in one of the recent updates? Is this to force people to use a keyboard binding after choosing a mouse button for a hotkey?

What's weird is that my DKs originally had Shift+MouseWheelDown as their mount...and it still shows that and works correctly even though it's no longer a valid key combination now. I even attempted to type in MouseWheelDown under key combination and it's a no go.