How to create a map if not exists or update it if it does in appsync

I have come across a scenario that incase a map is not created beforehand in dynamodb the update operation fails.

I am trying to create a map(userStatus in my case) incase it doesn't exist and also update it's key value in one pass.

here is the code from my appsync request template

{
"version": "2018-05-29",
"operation": "UpdateItem",
"key": {
"topicId": $util.dynamodb.toDynamoDBJson($ctx.stash.topicId)
},
"update" : {
"expression" : "SET userStatus = if_not_exists(userStatus, :emptymap), userStatus.#idx = :userContent",
"expressionNames": {
"#idx": "$ctx.args.input.event.payload.userId"
},
"expressionValues": {
":emptymap" : $util.dynamodb.toDynamoDBJson({}),
":userContent" : $util.dynamodb.toDynamoDBJson($userStatusContent)
}
}
}

i'm getting this error "Two document paths overlap with each other; must remove or rewrite one of these paths"
Here is a code that I tried but it fails

How can I solve this?

The relevant entry point is the AppSync UpdateItem request template shown in the issue; start by reading DynamoDB update-expression document-path rules and AppSync expressionValues handling. Reproduce the failing request with and without the userStatus map, then confirm a request-template expression that creates or updates the nested value without overlapping paths.

Written by the indexing model from the issue text.