Problem creating a hosted page with custom user fields

Problem creating a hosted page with custom user fields

When trying to create a hosted page via the subscription API with custom user fields, I am getting a 400 with the code 9062 and message "There is no label configured for the custom field index #."

The payload that I am sending to the API is as follows:
  1. array(4) { ["plan"]=> array(1) { ["plan_code"]=> string(3) "LGT" } ["addons"]=> array(6) { [0]=> array(1) { ["addon_code"]=> string(9) "RM-CW-A-1" } [1]=> array(1) { ["addon_code"]=> string(9) "RM-CE-A-1" } [2]=> array(1) { ["addon_code"]=> string(9) "RM-CB-A-1" } [3]=> array(1) { ["addon_code"]=> string(10) "RM-CSE-A-1" } [4]=> array(1) { ["addon_code"]=> string(8) "TKT-BLK1" } [5]=> array(1) { ["addon_code"]=> string(8) "LGT-CALL" } } ["redirect_url"]=> string() "######" ["custom_fields"]=> array(1) { [0]=> array(4) { ["index"]=> int(1) ["value"]=> string(1) "1" ["label"]=> string(7) "Project" ["show_in_pdf"]=> bool(false) } } }
In the subscription panel, I have added the custom field to all users in the Customer settings. I'm not quite sure what I'm missing.