How to get a nested JSON object (array) in AWS IoT-Core and send it to AWS timestream as a string?

Question

I have the following payload (JSON) in my MQTT broker AWS IoT-Core / Topic (+/+/myTopic):

{
  "id":"sig3678",
  "r_timestmp":1659443813,
  "sku":61,
  "z_x_1":{
     "a_b":[ 234, 245, ...],
     "c_d":[ 435, -45, ...]
  },
  "z_x_2":{
     "a_b":[ -123, 245, ...],
     "c_d":[ 678, 245, ...]
  },
  "ad_xz3":657
}

I have a rule that save this payload to a Timestream table:

SELECT id AS id, r_timestamp AS r_timestamp,cast(get("z_x_1","a_b") as string) AS z_x_1_a_b FROM '+/+/myTopic'

As per the documentation I can't use z_x_1.a_b to get the object [ 234, 245, ...]. Also I want to save a string from this object then I need to convert this object into a string using cast() function from this documentation.

Inside the rule I have the following: Dimensions: name: z_x_1_a_b, value ${z_x_1_a_b}

And in CloudWatch I am getting the followin error:

"failures": [
        {
            "failedAction": "TimestreamAction",
            "failedResource": "sig3678#z_x_1_a_b",
            "errorMessage": "Failed to write records to Timestream. The error received was 'Errors at dimensions.1: [Dimension value can not be empty.], Errors at measures.2: [Measure value is empty.]'. Message arrived on +/+/myTopic, Action: timestream, Database: sig3678, Table: z_x_1_a_b"
        }
    ]

The desired output should be:

{
  id:'sig3678',
  r_timestamp:1659443813,
  z_x_1_a_b: '[ 234, 245, ...]'
}

ready to be inserted in the table z_x_1_a_b. It seems that z_x_1_a_b is comming empty [Dimension value can not be empty.]. Maybe some issue with the way I am filtering the topic?!

What am I missing?