Scala help. DynamoDBMappingException: could not unconvert attribute

Question

I defined the following table in Scala. When I ran a test to query data from the dynamodb table, it throws exceptions:

com.amazonaws.services.dynamodbv2.datamodeling. DynamoDBMappingException

: TsuIntersectionRecord[comments]; could not unconvert attribute

The problem is with comments attribute, which is a list (SS). What should I do to fix unconert problem?

@DynamoDBTable(tableName = "tsu-overrides-table")
case class TsuIntersectionRecord(
  @(DynamoDBHashKey @beanGetter)(attributeName = "overrideId")
  @BeanProperty
  var overrideId: String,

  @(DynamoDBRangeKey @beanGetter)(attributeName = "intersection")
  @BeanProperty
  var intersection: String,

  @(DynamoDBAttribute @beanGetter)(attributeName = "runId")
  @BeanProperty
  var runId: String,

  @(DynamoDBAttribute @beanGetter)(attributeName = "snapshotDate")
  @BeanProperty
  var snapshotDate: String,

  @(DynamoDBAttribute @beanGetter)(attributeName = "tsuValue")
  @BeanProperty
  var tsuValue: Double,

  @(DynamoDBAttribute @beanGetter)(attributeName = "action")
  @BeanProperty
  var action: String,

  @(DynamoDBAttribute @beanGetter)(attributeName = "overrideValue")
  @BeanProperty
  var overrideValue: Option[Double],

  @(DynamoDBAttribute @beanGetter)(attributeName = "comments")
  @BeanProperty
  var comments: List[String],

  @(DynamoDBAttribute @beanGetter)(attributeName = "lastUpdateTime")
  @BeanProperty
  var lastUpdateTime: Int
) {
  def this() = this(null, null, null, null, 0.0, null, None, List(), 0)
}