 Terms whose function it is to refer can, on occasion, fail in that function. A use of the term “Vulcan” to refer to the planet orbiting between Mercury and the sun, fails to refer to anything for the reason that there is nothing for it to refer to. If I see the head of one dachshund protruding from behind a tree and the rear end of another protruding from the other side of the tree, and utter “that dog is huge”, my use of “that dog” has arguably failed to refer due to there being too many objects. It would seem that, by contrast, the term “I” cannot fail to refer either by there being too few or too many objects. “I” is guaranteed to refer.