Java DynamoDBTypeConverter未按预期工作

首先，正确运行的代码： DynamoDB表中的内容为：

"test_field" : "x:x:x"

实体POJO具有：

@DynamoDBTypeConverted(converter = SingleFieldConverter.class)
@DynamoDBAttribute(attributeName = "test_field")
private SingleField singleField;

使用适当的getter和setter方法。 转换器等级为：

public class SingleFieldConverter implements DynamoDBTypeConverter<String, SingleField> {

@Override
public String convert(SingleField singleField) {
   // removed for clarity
}

@Override
public SingleField unconvert(String s) {
   // removed for clarity
}

我的POJO反对：

@DynamoDBTypeConverted(converter = SingleFieldConverter.class)
@DynamoDBAttribute(attributeName = "test_field")
private List<SingleField> singleField;

我在网上找到的类似例子据称是有效的，但我的例子却没有。我错过了什么，请帮忙

@DynamoDBTypeConverted(converter = SingleFieldConverter.class)
@DynamoDBAttribute(attributeName = "test_field")
private List<SingleField> singleField;

public class SingleFieldConverter implements DynamoDBTypeConverter<String, List<SingleField>> {

@Override
public String convert(List<SingleField> singleField) {
   // removed for clarity
}

@Override
public List<SingleField> unconvert(String s) {
   // removed for clarity
}

DynamoDBMappingException
. . . . . [test_field]; could not unconvert attribute
 . . .Caused by: com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMappingException: expected S in value {L: [{S: x:x:x:,}, {S: x:x:x:,}],}