"Cannot unify set with unit" error

Question

I am trying to write a simple contract in which I am using set of records as storage.

type data = {
    message : string,
    did: address
};

type parameter =
| ["Published"]
| ["Revoked"];

type storage = set<data>;
type return_ = [list <operation>, storage];
let store: storage = Set.empty;

/* Two entrypoints */
const published = ([store] : [storage]) : storage => {
    const entry = {message: "Credentials for has been published to the blockchain", did: Tezos.get_source()};
    Set.add(entry, store);
    return store;
};
const revoked = ([store] : [storage]) : storage => {
    const entry = {message: "Credentials for has been revoked", did: Tezos.get_source()};
    Set.add(entry, store);
    return store;
};

But I am getting the below error in line

Set.add(entry, store)

Does anyone know what is issue here ?

Invalid type(s) Cannot unify set (record[did -> address , message -> string]) with unit.