Full Idea
In Frege's account sentences such as 'Pegasus does not exist' will be neither true nor false, since the truth-value of a sentence is its referent, and the referent of a complex expression is determined by the referent of its parts.
Gist of Idea
Frege's compositional account of truth-vaues makes 'Pegasus doesn't exist' neither true nor false
Source
Sarah Sawyer (Empty Names [2012], 2)
Book Reference
'Routledge Companion to Phil of Language', ed/tr. Russell/Graff Faria [Routledge 2015], p.154
A Reaction
We can keep the idea of 'sense', which is very useful for dealing with empty names, but tweak his account of truth-values to evade this problem. I'm thinking that meaning is compositional, but truth-value isn't.
Related Idea
Idea 18937 If sentences have a 'sense', empty name sentences can be understood that way [Frege, by Sawyer]