Might possbly follow the logic of the current English police caution
You do not have to say anything. But it may harm your defence if you do not mention when questioned something which you later rely on in court.
In other words, compelled to attend as a witness, not compelled to testify but simply advised that silence may come back to bite them.
The caution does not apply to witnesses, only suspects in The UK. No one in this country can be forced to give a witness statement, however they can be compelled to attend court and give evidence on oath.