What is causing the error "AmazonDynamoDBException: One or more parameter values are not valid" during a POST /entities update

Question

We are performing the following update

POST https://dev.reltio.com/reltio/api/<>/entities/

[{
"attributes": {
"Country": [
{
"value": "AU"
}
]
},
"crosswalks": [
{
"contributorProvider": "false",
"dataProvider": "true",
"type": "configuration/sources/Reltio",
"value": ""
},
{
"contributorProvider": "true",
"dataProvider": "false",
"type": "configuration/sources/IMSPLAN",
"value": "0012F00000uLZ0PQAW"
}
],
"type": "configuration/entityTypes/HCP"
}]

This results in an error.

[
    {
        "index": 0,
        "errors": {
            "severity": "Error",
            "errorMessage": "Database access error",
            "errorCode": 1001,
            "errorDetailMessage": "Internal error occurred while operating with database, message - [Message: com.amazonaws.services.dynamodbv2.model.AmazonDynamoDBException: One or more parameter values are not valid. The AttributeValue for a key attribute cannot contain an empty string value. Key: id (Service: AmazonDynamoDBv2; Status Code: 400; Error Code: ValidationException; Request ID: K02D0UOOL9G9F87NRQ27JO06RVVV4KQNSO5AEMVJF66Q9ASUAAJG). Requests: [{RequestItems: {dev_VjhOcyiats2DB93_ENTITIES={Keys: [{id={S: ,}}],AttributesToGet: [id, tp, dl, ln, vr, ut, rut, fl],ConsistentRead: true,}},ReturnConsumedCapacity: TOTAL}].]. Please view application log for details"
        },
        "successful": false
    }
]

Answer

DynamoDB has a restriction where empty string keys/values are not allowed.