Full Idea
Fregeans tend to treat as a fundamental tenet that sense determines reference; same sense, same reference. From that it follow trivially that indexicals don't have the same sense: different uses of 'I' have different referents, so sense must differ.
Gist of Idea
Fregeans say 'I' differs in reference, so it must also differ in sense
Source
Cappelen,H/Dever,Josh (The Inessential Indexical [2013], 04.6)
Book Reference
Cappelen,H/Dever,J: 'The Inessential Indexical' [OUP 2013], p.72
A Reaction
Interesting. Since it seems implausible that 'I' is profoundly different when two people use it, this seems to be a strong argument against Frege's distinction. But I rather like Frege's distinction, while being sceptical about 'I', so I'm baffled....