A thought: When I run out of shield, I love the handy warning message. But then when I hit 's', and successfully activate a shield, the warning message is still displayed until it times out. This might just be me, because I'm old and sick, but it makes me uncertain whether I now have any shields activate or not.
I know there are other indications that my shield invoke really did work. and it never lasts for long - it's only a second or so. But when I've just lost my last shield, it's at a critical moment when I'm probably already under pressure and flustered, so I get uncertain and distracted, which is sometimes fatal.
So I'm wondering whether that warning message should disappear instantly when you activate a shield.