Well, it's not a bug, but not a feature :)
Actually, this input is energy spent per second (I will add this description)
So, if you try to set 36 energy into it, that means that you would have to spend this amount each second. But, if you put way too large number into this input, you will spent all your energy during a second or so, and this can cause loosing your creatures.
So, this is kind of limitation added as very quick workaround to avoid loosing time by making some typos, but I will change how it's implemented (mb, will just add confirmation with warning that you are about to spend all your energy if you are going to have negative balance of it)