Error occured when using UDF with scoped notebook

java.lang.ClassCastException: cannot assign instance of scala.collection.immutable.List$SerializationProxy to field org.apache.spark.rdd.RDD.org$apache$spark$rdd$RDD$$dependencies_ of type scala.collection.Seq in instance of org.apache.spark.rdd.MapPartitionsRDD

import java.text.SimpleDateFormat

def diffHour(s1: String, s2: String): Long = {
  var hour = 0L
  try {
    val sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss")
    val d1 = sdf.parse(s1)
    val d2 = sdf.parse(s2)
    hour = d2.getTime - d1.getTime
    hour /= 1000 * 60 * 60
  } catch {
    case e: Exception => hour = -1
  }
  hour
}

sqlContext.udf.register("diffHour", diffHour _)

%sql
SELECT
  id,
  time,
  diffHour(time, '2019-01-01 00:00:00') as hour
FROM users