who or what can be a protagonist?

I recently had a writing expert tell me a robot cannot be a protagonist. Earlier this expert said God can't be a protagonist. Only now do I understand what he was getting at, namely, a protagonist must have something to lose. There must be something on the line. There must be the possibility that he/she/it can fail. This is hard to imagine with the Judeo-Christian G-O-D, omniscient and omnipotent. However, I have read stories with God as the protagonist; this God could fail, so I guess technically it was not The Judeo-Christian God. Similarly, the robot in question was sentient, it had intelligence, feelings/emotions, hopes/dreams, and could die. So, I say this robot could also be a protagonist. Can a random non-sentient machine be a protagonist? No. Can a ghost or other supernatural creature be a protagonist? Yes, if he/she/it can act and had something to lose.
What do you think?

For more on my take on protagonists see: P is for Protagonist.