Why mutation queries are falling into 'success' instead of 'onError'?

const signUpMutation = useMutation({
    mutationFn: ({signUpForm: signUpForm}) => signUpRequest(signUpForm),
    onSuccess: (result, variables, context) => {
      if(result.status && result.status === 400) {
        // Should go to onError and goes here.
      } else {
      }
    },
    onError: () => {
      showErrorToastMessage("The registration isn't allowed. ")
    },
  });


export const signUpRequest = async (formData) => {
  return await fetch('/api/auth/signup', {
    method: 'POST',
    headers: {
      Accept: 'application/json',
      'Content-Type': 'application/json',
    },
    body: JSON.stringify(formData),
  }).then(r => r.json());
};