API: Creating Sales Orders - "Invalid value passed for JSONString"

API: Creating Sales Orders - "Invalid value passed for JSONString"

Hello all,

I am trying to create a sales order from API in ZOHO inventory and I keep obtaining 
  1. {
        "code": 4,
        "message": "Invalid value passed for JSONString"
    }
I tried many combinations, but the same error appear.

Yes, I can obtain all sales orders  and all items.

For creating a sales order I use:
- the url:
- the JSON:
  1. {
        "customer_id": 235260000000310001,
        "salesorder_number": "SO-00009",
        "custom_fields": [
            {}
        ],
        "reference_number": "0905219",
        "line_items": [
             {
                "item_id": 235260000000476073,
                "quantity_invoiced": 2,
                "sub_total": 444,
                "tax_total": 29,
                "total": 550
             }
         ]
    }
Does anyone have any idea about what am I doing wrong?  Or am I missing something ?

Thank you for your help.

Best Regards,
Mihaela