Carnap's diagonal lemma asserts that for every computable formula **f**, accepting a natural number as argument and resolving to {false,true}, there exists a logic sentence **s** for which: 

s $\leftrightarrow$ f($\ulcorner s\urcorner$)

With $\ulcorner s\urcorner$ the Gödel encoding for logic sentence s.

I have a problem with the example function f($\ulcorner s\urcorner$) = $\neg$s. It looks like according to Carnap's diagonal lemma, there exists a logic sentence **s** for which the following could hold:

s $\leftrightarrow$ $\neg$s

This is obviously, in one way or another, a wrong interpretation of what the lemma says or implies, but I cannot explain why it is wrong ... What's wrong?