The Objection from Cognitive Significance

Here's the objection:

1. If Millianism is true, and ‘Ice-T’ and ‘Tracy Lauren Marrow’ co-refer, then (i) and (ii) encode the same proposition:
(i) Ice-T is Ice-T
(ii) Ice-T is Tracy Lauren Marrow.
2. The proposition encoded by (i) is uninformative, true in virtue of meaning (analytic) (like ‘all bachelors are unmarried males’), is knowable without empirical investigation (a priori), cannot be rationally disbelieved, and the proposition encoded by (ii) is not (any of those things).
3. If (2), then it’s not the case that (i) and (ii) encode the same proposition.
4. So it’s not the case that (i) and (ii) encode the same proposition.
5. So Millianism is false or ‘Ice-T’ and ‘Tracy Lauren Marrow’ don’t co-refer.
6. ‘Ice-T’ and ‘Tracy Lauren Marrow’ co-refer.
7. So Millianism is false.

Is this argument sound? If not, which premise might a Millian reject? If it is sound, then Millianism is incorrect and we need a different account of the propositions encoded in (i) and (ii). More generally, we'll need an account of the semantic content of proper names that is not vulnerable to this problem.

Thoughts?