Spark UDAF in examples fail with not serializable error

The type-safe example given at https://spark.apache.org/docs/latest/sql-ref-functions-udf-aggregate.html fails with a not serializable exception Is this a known issue?

Example UDAF fails with "not serializable" exception

https://spark.apache.org/docs/3.3.2/sql-ref-functions-udf-aggregate.html I'm trying to run this example on Databricks, and it fails with the stacktrace below. It's literally a copy-paste from the example, what am I missing? Job aborted due to stage failure: Task not serializable: