How to deal with the abnormal extraction time of the calculated column？

[forideal]

Hi My Friends：

    How to deal with the abnormal extraction time of the calculated column ？

    My DDL ：

    CREATE TABLE mytable (

      xx string,

      t as get_event_time_from_xx(xx),

      WATERMARK FOR t AS t - INTERVAL '1' SECOND

   ) WITH (xxx)

   If an exception occurs inside my function and I don’t want to generate a default time or an error time, how should I deal with it?

   Looking forward to your answer, thank you very much.

Best,

forideal