Canceling color selection for Command Tower leaves it tapped rather than auto untapping

I go to pay for a spell which needs two colors, say {G}{W}. My command tower can produce both {G} and {W}. It will ask me for which one to pick.

If instead if picking a color I hit cancel the command tower will stay tapped and I will still be in the middle of pay for the spell.

What I would expect to happen is for the command tower to untap itself. Instead it stays tap, but no mana is created.