Adding invoice_items on create new invoice results in 404

Hi - whatever I try with adding invoice_items to an invoice results in 404 - resource not found. If I leave them off, I get an invoice created perfectly (but obvs with no line items).

I’m using Guzzle to send the request, which is built from an array like this:

$dat['invoice_items'][]=array('description'=>'One time setup','price'=>"1000");

which results in a payload like this:

"invoice": {
		"status": "draft",
		"dated_on": "2017-12-22",
		"payment_terms_in_days": 0,
		"invoice_items": [{
			"description": "One time setup",
			"price": 10.95
		"contact": "{someuri}"

No issues with auth etc, that’s all working fine (merrily creating contacts etc with same guzzle code)

Please advise things to try…