Full Idea
Two features of natural languages cause them to violate bivalence: singular terms (or proper names) which have a sense but fail to denote an object ('the centre of the universe'); and predicates which are not well defined for every object.
Clarification
Bivalence says sentences can only be 'true' or 'false'
Gist of Idea
Language can violate bivalence because of non-referring terms or ill-defined predicates
Source
Michael Dummett (Thought and Reality [1997], 4)
Book Reference
Dummett,Michael: 'Thought and Reality (Gifford Lectures)' [OUP 2006], p.47
A Reaction
If we switch from sentences to propositions these problems might be avoided. If there is no reference, or a vague predicate, then there is (maybe) just no proposition being expressed which could be evaluated for truth.