Amazon web services 如何使用Golang filter builder筛选空值？

继续，我正在努力复制Go代码以筛选特定列的空值：

import (
        "github.com/apex/log"
        "github.com/aws/aws-sdk-go-v2/aws"
        "github.com/aws/aws-sdk-go-v2/aws/endpoints"
        "github.com/aws/aws-sdk-go-v2/aws/external"
        "github.com/aws/aws-sdk-go-v2/service/dynamodb"
        "github.com/aws/aws-sdk-go-v2/service/dynamodb/expression"
)

// aws --profile dd dynamodb scan --table-name plocal2 --filter-expression 'paymentmethod = :null' --expression-attribute-values '{":null" :{"NULL":true}}'

var table = "plocal2"

func main() {
        cfg, err := external.LoadDefaultAWSConfig(external.WithSharedConfigProfile("dd"))
        if err != nil {
                log.WithError(err).Fatal("setting up credentials")
                return
        }
        cfg.Region = endpoints.ApSoutheast1RegionID
        db := dynamodb.New(cfg)
        filt := expression.Name("paymentmethod").Equal(expression.Value(expression.Null))
        expr, err := expression.NewBuilder().
                WithFilter(filt).
                Build()
        if err != nil {
                log.WithError(err).Error("failed to build expression")
                return
        }
        scanReq := db.ScanRequest(&dynamodb.ScanInput{
                ExpressionAttributeValues: expr.Values(),
                FilterExpression:          expr.Filter(),
                TableName:                 aws.String(table),
        })
        result, err := scanReq.Send()
        if err != nil {
                log.WithError(err).Error("failed to query table")
                return
        }
        log.Infof("%d", len(result.Items))
}

错误是：

2019/04/30 16:59:54 error failed to query table     error=ValidationException: Invalid FilterExpression: An expression attribute name used in the document path
 is not defined; attribute name: #0
        status code: 400, request id: 8CFTOAGVB7M1IM3DC6BFPEVLUNVV4KQNSO5AEMVJF66Q9ASUAAJG

有没有关于如何调试的技巧，或者使用

真希望有，因为空字符串变为空字符串时，对我来说，空字符串过滤应该是一个非常常见的用例