How can I deliberately fail a mutation for testing purposes?

For example in the following code, how do I make the code go into the ```.catch``` block

```
changeSubjectLine({
variables: {
campaignId,
},
})
.then(() => {
toast(successToast('Subject line dynamic saved.'));
})
.catch((e) => {
toast(errorToastConfig(e));
// I want the code to go into here
});

```
Is there a flag I can put in there to make it do that

The issue names no files or tests; begin with the changeSubjectLine mutation call and its promise chain. Determine whether the client or Apollo tooling exposes a testable failure hook, then verify that the .catch callback receives an error.

Written by the indexing model from the issue text.