Full Idea
The predicate 'is a prime number greater than all other prime numbers' is necessarily not true of anything, but it is not semantically defective, for it occurs in sentences that constitute a sound proof that there is no such number.
Gist of Idea
References to the 'greatest prime number' have no reference, but are meaningful
Source
Timothy Williamson (Vagueness [1994], 6.2)
A Reaction
One might reply that the description can be legitimately mentioned, but not legitimately used.
Book Reference
Williamson,Timothy: 'Vagueness' [Routledge 1996], p.168