getting { code: 120121, message: 'Invalid Custom field index provided.' } while creating contact in zoho inventory

{
        contact_name: String(mobileNumber),
        custom_fields: [
          {
            value: "",
            index: 1,
          },
          {
            value: "",
            index: 2,
          },
          {
            value: "",
            index: 3,
          },
        ],
      }

This is my custom_fields
Please can anyone tell why I am getting this error.

Hoping for fast reply, thanks in advance.