policy to limit dynamodb UpdateItem with a condition expression

Hi,

I have a dynamodb table with the following attributes:

1. mykey: partition key
2. myversion: a mutable number
3. mymap: a mutable map
4. several other attributes

I have an application that should call the following dynamodb UpdateItem request:

1. a conditional update expression that verifies the existing value of myversion attribute: myversion = 123
2. an update expression that modifies mymap attribute: SET mymap = {...}

I am trying to write a fine-grained iam policy that allows an application to perform this UpdateItem with minimal permissions:

1. the partition key must be equal to a predefined value, e.g. "part123"
2. only "mymap" attribute must be modifiable by the application
3. all attributes can be read by the application

Tried this policy:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "dynamodb:UpdateItem"
            ],
            "Resource": [
                "arn:aws:dynamodb:*:<ACCOUNT>:table/<TABLE>"
            ],
            "Condition": {
                "ForAllValues:StringEquals": {
                    "dynamodb:LeadingKeys": [
                        "part123"
                    ],
                    "dynamodb:Attributes": [
                        "mykey",
                        "mymap"
                    ]
                }
            }
        }
    ]
}

It does work if the UpdateItem is called without a conditional expression on myversion. But once I add the conditional expression it fails with the following error:

AccessDeniedException: User: [...] is not authorized to perform: dynamodb:UpdateItem on resource: [arn:aws:dynamodb:...] because no identity-based policy allows the dynamodb:UpdateItem action

Adding the conditional expression attribute myversion under "dynamodb:Attributes" section in the policy makes it work but I'm afraid this also allows updating myvesion itself, which is unwanted.

How can I define a policy that allows dynamodb UpdateItem to modify only a specific field while referencing other fields in the conditional expression?

thanks