And then, since the type NIL is a subtype of all types, it's a subtype of CHARACTER. So because the type STRING is the union of all array types whose element type is a subtype of CHARACTER, an array that can't store any values is also a string. Oops.

(Also, just for onlookers, in ANSI Common Lisp, but not its ancestors or its sorta-sibling Emacs Lisp, characters are disjoint from integers. That's why the intersection of BIT and CHARACTER is empty.)