1,760. Statements That Don't Cancel a Get
Hilchos Geirushin 6:23
If a man says, "This get isn't a get," "It's unacceptable," "It's ineffective," "It doesn't permit," "It doesn't release," "It doesn't send out," "It doesn't divorce," or "It's a shard," his words are meaningless because they don't convey that the get should be canceled. Rather, he's making a statement of fact and that statement happens to be inaccurate. It's like saying that something prohibited is permitted or that something ritually unclean is clean (i.e., mistaken and meaningless).
Hilchos Geirushin 6:24
If he says, "This get is voided," the implication is the past tense as in Shir HaShirim 5:6, "He turned and was gone." There is therefore a doubt in such a case and, if such a get is used, the divorce is in doubt.