How to use input transformer for ECS Fargate launch type with Terraform CloudWatch event trigger

I'm using terraform to create a CloudWatch Event Trigger with a ECS Fargate launch type which the event source is S3. When I use the input_transformer field to pass in the bucket and key into the ECS task, my event rule results in a failed invocation.

This is the aws_cloudwatch_event_rule:

resource "aws_cloudwatch_event_rule" "event_rule" {
  name          = "dev-gnss-source-put-rule-tf"
  description   = "Capture S3 events on uploads bucket"
  event_pattern = <<PATTERN
{
  "source": [
    "aws.s3"
  ],
  "detail-type": [
    "AWS API Call via CloudTrail"
  ],
  "detail": {
    "eventSource": [
      "s3.amazonaws.com"
    ],
    "eventName": [
      "PutObject"
    ],
    "requestParameters": {
      "bucketName": [
        "example-bucket-name"
      ]
    }
  }
}
PATTERN
}

This is the aws_cloudwatch_event_target:

resource "aws_cloudwatch_event_target" "event_target" {
  target_id = "dev-gnss-upload-event-target-tf"
  arn       = "example-cluster-arn"
  rule      = aws_cloudwatch_event_rule.event_rule.name
  role_arn  = aws_iam_role.uploads_events.arn
  ecs_target {
    launch_type = "FARGATE"
    task_count  = 1 # Launch one container / event
    task_definition_arn = "example-task-definition-arn"
    network_configuration {
      subnets         = ["example-subnet"]
      security_groups = []
    }
  }

  input_transformer {
    input_paths = {
      s3_bucket = "$.detail.requestParameters.bucketName"
      s3_key    = "$.detail.requestParameters.key"
    }

    input_template = <<TEMPLATE
{
  "containerOverrides": [
    {
      "name": "myproject-task",
      "environment": [
        { "name": "S3_BUCKET", "value": <s3_bucket> },
        { "name": "S3_KEY", "value": <s3_key> }
      ]
    }
  ]
}
TEMPLATE
  }
}

If I remove the input_transformer section, it will work fine, but I need to pass in the s3 bucket and key to process the particular file.

My rationale for doing this is to remove the need for an intermediary Lambda and was guided by this Medium post: https://medium.com/@bowbaq/trigger-an-ecs-job-when-an-s3-upload-completes-3559c44c37d1

Any advice is appreciated.