Full Idea
In supervaluations, the Law of Identity has no value for empty names, and remains so if extended. The Indiscernibility of Identicals also fails if extending it for non-denoting terms, where Fa comes out true and Fb false.
Gist of Idea
Identities and the Indiscernibility of Identicals don't work with supervaluations
Source
Stephen Read (Thinking About Logic [1995], Ch.5)
Book Reference
Read,Stephen: 'Thinking About Logic' [OUP 1995], p.142