How to create multiple item lines with zohobooks API

Im trying to create an invoice and I have multiple item lines but I couldnt find where to do that in the documentation

'line_items' => [
    [
        'item_id'           => '7131',
        'description'       => something
        'rate'              => somthing
        'quantity'          => something
    ]
]

I have something like this for the first line and it works just fine but when I try to add another one it doesnt even if I rename it to line_items2 it gives me this error:

resulted in a 400 Bad Request response: {"code":15,"message":"Please ensure that the line_items2 has less than 100 characters

This is super urgent help.