DynamoDB update map value only if versions match

Question

I have the following schema:

{
    balance: {
      customData: string,
      version: number
    }
}

I need to update balance.customData only if version == 2. Once update operation succeeds, I need to increment version by 1.

I tried the following update transaction

  Update: {
    Key: {
      'pk': pk,
      'sk': sk
    },
    TableName: this.config.getTable(),
    UpdateExpression: 'SET #balance.#customData = :customData, #balance.#version = #balance.#version + :incr',
    ConditionExpression: '#balance.#customData.#version = :version',
    ExpressionAttributeNames: {
      '#customData': 'customData',
      '#balance': 'balance',
      '#version': 'version'
    },
    ExpressionAttributeValues: {
      ':customData': balance.customData,
      ':version': balance.version,
      ':incr': {'N': 1},
    }
  }

But when I run this, I get ValidationException: Invalid UpdateExpression: Incorrect operand type for operator or function; operator or function: +, operand type: M I assume I'm not providing correct syntax to increment map value.

Even if I remove increment part

  Update: {
    Key: {
      'pk': pk,
      'sk': sk
    },
    TableName: this.config.getTable(),
    UpdateExpression: 'SET #balance.#customData = :customData,
    ConditionExpression: '#balance.#customData.#version = :version',
    ExpressionAttributeNames: {
      '#customData': 'customData',
      '#balance': 'balance',
      '#version': 'version'
    },
    ExpressionAttributeValues: {
      ':customData': balance.customData,
      ':version': balance.version
    }
  }

I still get ValidationError with reason Other, which is not very helpful. Can you please help me understand what I'm doing wrong? Thanks